Sanchez looks like the starter

Coach Pete Carroll refuses to say it officially, but sophomore quarterback Mark Sanchez will almost certainly start for USC for the first time Saturday when the 10th-ranked Trojans play Arizona at the Coliseum.

On Wednesday, Sanchez took first-team snaps for the third consecutive day while John David Booty rested his broken finger.

Booty said the middle finger on his right hand swelled after he threw about 50 passes Tuesday, and that he did not want to set himself back by trying to return too soon.

“I don’t want to force and press it and here I’m out two, three weeks because of something I could have been out only a week,” Booty said.

Another issue of concern is whether there will be enough sound linemen to protect whomever plays quarterback.

Backup linemen Charles Brown and Thomas Herring joined the injured list with ankle sprains suffered Tuesday and both said they were doubtful for Saturday’s game. Brown, Herring, centers Kris O’Dowd (knee) and Matt Spanos (lacerated fingers) and guard Chilo Rachal (knee) watched practice from the sideline.

“I don’t think it’s ever been this bad,” senior tackle Sam Baker said. “It’s like an injury virus or something.”

The first unit Wednesday featured Baker and Drew Radovich at tackle, Zack Heberer and Alatini Malu at guard and Jeff Byers at center.

Butch Lewis and converted defensive linemen Alex Parsons and Michael Reardon got plenty of work as backups.

“I’ve got tough guys, they’ll be fine,” offensive line coach Pat Ruel said.

Sanchez also is confident that he will be prepared.

“The guys had a little more chemistry today with me and it’s only going to get better,” he said.

Booty is not concerned that sitting out might allow Sanchez to make it tough to reclaim the starting job.

“I’m really not worried about that,” Booty said. “I just want our team to play good football, that’s the bottom line. I want us to win, and if I’m not in there I think Mark gives us the best shot to do that. And I’m looking for him to play well if his number is called.”

Senior quarterback Michael McDonald has been taking second-team snaps in case Sanchez is ineffective or injured and Booty is unavailable.

“I probably have to watch a little more film this week than I have in the past, but I’ll be ready if they need me,” McDonald said.

Tailback Stafon Johnson (foot) said he was “about 80%” healthy and that he would play Saturday if he felt “90%.”. . . Asked who would be the No. 2 tailback behind starter Chauncey Washington, Carroll said, “I think we’re ready to go with Hershel [Dennis] in all situations and Desmond [Reed] can do everything we need to do. We still have Joe [McKnight] and A.B. [Allen Bradford]. They are ready to play for us and we want those guys to be part of it.”. . . Linebacker Thomas Williams (ankle) practiced and will play against Arizona, according to Carroll.

gary.klein@latimes.com