Q. T Lane Johnson said he played left tackle in practice yesterday. Will he play there if T Jason Peters can't go on Sunday?

COACH KELLY: We'll determine that after we get through the week of training, just like we always talk about. None of the decisions on where we're playing will happen until after Saturday's training.

Q. Since you haven't previously opted to use Johnson at left tackle, does this mean you are more comfortable with using him there, now?

COACH KELLY: No. Again, we'll wait till Saturday to make that determination. He got a couple reps yesterday.

Q. Has he taken more snaps at left tackle this week than usual?

COACH KELLY: Well, usually he takes zero. So, yes, he's taken more snaps than zero.

Q. Will part of that decision be based on the matchup with Dallas?

COACH KELLY: No. Just trying to figure out what's going to happen with Jason and whether he's going to go or if he's not going to go.

Q. How is Peters coming along?

COACH KELLY: He's working on his rehab, so we'll see where he is. Again, we won't determine anything until Saturday.

Q. Is he practicing today?

COACH KELLY: He's not going to practice today.

Q. We talked to WR Nelson Agholor yesterday and he seemed kind of disappointed about his performance before he got hurt. What is your take on what he's done as a rookie coming in and everything he's done so far?

COACH KELLY: I think he's done a nice job getting acclimated in terms of what we're doing. I think the injury has set him back. He was starting to kind of pick it up. He had a couple big catches in the Washington game for us: the one-hander on the post route and caught a big out route on our sideline. Then he got banged up. I think he was just starting to kind of come in, get real comfortable with how the league is and what the matchups are each week and the challenges that he's going to face each week. Then he got hurt. We're anxious to get him back. He's out there training with us now. He went through everything yesterday. Going to go back through and see what he can do today. I think he's going to be able to go through everything today and then we'll make an assessment with him on Saturday, also.

Q. You've been running the ball more effectively out of 12 personnel than 11 personnel this season. Does that surprise you at all? Why do you think that is?

COACH KELLY: I just think it depends on how defenses are deploying and how you use 12. Some people stayed -- I think Carolina stayed in nickel versus it. It really depends on the matchup. That's why we go through that 12-11 conversation all the time as a coaching staff. What are we going to get out of them and how are they going to defend it and what does that mean to us? So it's really about how people are defending it.

Q. Not during the draft, but after the draft, what kind of interest did you guys have in Cowboys rookie OL La'el Collins?

COACH KELLY: We didn't have any interest.

Q. RB Ryan Mathews was back on the practice injury report with a groin injury. How did that happen?

COACH KELLY: It happened in the Carolina game.

Q. So he had a groin on Tuesday. He did nothing in practice on Wednesday.

COACH KELLY: Yeah. And his injury is the injury I told you guys about in the Carolina game, where we didn't feel like he was going. So he stayed and got treatment over break and was here on Tuesday. Now we'll see how he is and monitor him the rest of the week.

Q. There was a more defined difference between left tackle and right tackle a decade ago. Are those positions more similar now?

COACH KELLY: No, I mean, there's a difference between playing the left side and the right side. I don't know [the difference between] 10 years ago compared to this year, [but] I think there's a difference between the two positions. A lot of the pass rushers are on the defensive right or the offensive left, in terms of how you face them, because it's backside of most quarterbacks, unless you're [left handed] like [former 49ers QB] Steve Young or [former Eagles and current Steelers QB] Michael Vick. Then it's the other way around. I still think there's a difference between a right tackle and a left tackle.

Q. What are the differences?

COACH KELLY: Stance, pass rushers that you're going to face and the ability to operate your feet in a different manner. But really matchups, I think, is really what it comes down to.

Q. What have you seen defenses do, if anything, differently when RB Darren Sproles has come out in passing situations?

COACH KELLY: As a running back, we get a lot more doubling [of] the back. That's what you kind of see, where teams that play man-free, really the free player is monitoring Darren, along with the other guy that's responsible for covering him.

Q. Was that a lot of what you saw increasingly last year when his passing numbers --

COACH KELLY: Yeah, that's what we saw last year. Most of the time we saw Darren in there, people were trying to double him, and rightfully so, because he's the weapon that you try to take away in the passing game.

Q. That will open things up for other guys, right? If they are going to double Sproles.

COACH KELLY: Yeah, but they're still covering the other guy. It's not like it's a zone. I mean, everybody else is getting single coverage. It was just that sometimes there's a low-hole player that's not free to roam and help anywhere. He's actually specifically assigned to help with him.

Q. Since Cowboys DE Greg Hardy has been inserted into the lineup, how much has Dallas moved him around?

COACH KELLY: They are moving him around. You'll get him inside and get him in some matchups, but predominantly he's been at the strong-side outside, defensive end for them. But they have moved him around a little bit. I don't have the exact percentages, but he's played predominantly at the defensive end spot, and he's made a lot of damage and hay there. But they have moved him around, so you've got to be aware that he's not always going to line up across from your left tackle; he could be inside on the guards. He's been out at the other defensive end also. It's predominantly in one spot, but there is movement and you have to be aware of where he is.

Q. Just in general, when you try to build the program you want to build here, how do you evaluate a guy who maybe has some things in his past that another team might look at and say, "I don't want that here?"

COACH KELLY: We look at everybody individually, just what happened, or if there was a transgression, what they went through and how it happened, and try to get to the bottom of it so you can make an assessment in terms of how he'll fit on your team. So everybody is dealt with on an individual basis.

Q. How effective is T Dennis Kelly at guard compared to tackle?

COACH KELLY: I think he's the -- I don't know if you would say the same, but I think he's done a nice job because he's gone in at guard for us. That's where we've seen him, and [we] haven't seen him unless it was preseason games at tackle. It's kind of tough when you see him in a preseason game. Who's he running against? Is he running against their threes when he's out there at the right tackle spot? What we've seen him against in terms of good, live competition has been at guard. We think he's done a nice job when he's been in at guard.

Q. Did RB DeMarco Murray's health hold him back earlier in the season?

COACH KELLY: DeMarco Murray's health? I don't think so, no.

Q. Offensive Coordinator Pat Shurmur suggested yesterday that Murray is healthier now.

COACH KELLY: That's a good question for DeMarco. He's been up for every game and active for every game. He missed the Jets game obviously. That's a question for him in terms of what he says on that one. We didn't look at it when the games he was active [like] 'Let's limit his carries because we don't think he's healthy.' He was healthy enough to go in all those games. He was good to go.

Q. Is there a lot of value in the first Dallas tape for you on offense or do you not go there?

COACH KELLY: No, we go there. There's value in any tape. We've watched every game they've played. So we don't look at one game and discount that compared to another game. You've got a whole body of work, seven games so far, that you've seen on them. We watch all seven games, break down all seven games, and use the information from all seven games.

Q. When you look at the quarterback situations around the league, 49ers QB Colin Kaepernick just got benched, Broncos QB Peyton Manning struggling a little bit. What do you think that says about the volatility of the position? How important is it to have a pretty stable quarterback you can rely on?

COACH KELLY: I don't think it has anything to do with this year. I think you can say that for every year this league's been in existence or the game of football has been in existence. The focal point of what you're doing is the quarterback spot. It's the way the game is. I mean, that's the guy that has the ball on every play and [it's] how he distributes it, whether it's to the right guys or to the wrong guys. Are you moving your team down the field and managing the game the way he's supposed to manage a game? That's the key. Very rarely is a team successful that doesn't have really good quarterback play. There's been some exceptions where it was just a defense for the ages and the offense was just kind of – you're winning a game 7-0, or 6-3 or whatever. That's the deal. Most of the time, the percentages are that when you get good quarterback play, you probably have a good football team.

Q. How would you evaluate the guys like Kaepernick and Redskins QB Robert Griffin III who have struggled?

COACH KELLY: I'm really honestly worried about the Cowboys. I haven't watched a snap of the 49ers or know what's going on with anybody else. I mean, you can see reports of guys doing all this other stuff. But I'm more concerned with [Cowboys DE] Greg Hardy than we are with Colin Kaepernick or RGIII.

Q. A guy like Kaepernick led the 49ers to the Super Bowl. Colts QB Andrew Luck and Manning have had a ton of success. To see them struggling --

COACH KELLY: [Former NFL QB] Clint 'Long Bomb' Longley came in and did an unbelievable job for the Dallas Cowboys at one point in time, too. Again, I'm worried about the Cowboys, not anybody else's quarterback spot. Some guys have come in and had great years and been one-year guys, and other guys don't play until later in their career like [former NFL QB] Kurt Warner that sat and was playing Arena League ball and everybody was saying what about him, then turns into a Hall of Famer. So how it happens -- I'll let some people write books about that. But that's not what my focus and attention is on. My focus and attention is on blocking Greg Hardy and a really good front and trying to get people to get [Cowboys QB] Sean Lee straightened out and making sure we're sound on defense in terms of facing [Cowboys QB] Matt Cassel, [Cowboys WR] Dez Bryant and [Cowboys TE] Jason Witten.

Q. Would you have put up with what it took to get Greg Hardy on the field?

COACH KELLY: Again, I have no idea what it took for them to do and we weren't involved in that. That's their deal.

Q. If you think back to March, how did you sell the backfield combination to Murray and Mathews?

COACH KELLY: We didn't sell anything. We were just very honest in terms of what we're doing, and we were bringing two guys in here and those were the two guys we wanted, so... Explained it straight out. They knew that they were both here. They were actually both here at the same time, so they knew exactly what was going on. I don't think there is any selling job that has to go on. It's just, 'This is what we do, here is the offense that we run and these are the guys we want to have be a part of it.' They both liked it.