Backup Cowan out indefinitely

UCLA quarterback Patrick Cowan will miss “several” weeks after suffering a partially torn hamstring in his left leg late in Monday’s practice.

Cowan underwent an MRI Tuesday afternoon, which revealed the tear.

“I’m not bummed, I’m sunburned,” joked Cowan, a junior who started eight games after Ben Olson was injured last season. “I need a bucket hat out at practice.

“It just makes me think how fast you need to be running to tear a muscle. So I guess I was running pretty fast on the play.”

McLeod Bethel-Thompson, a walk-on, will be the backup quarterback when the Bruins open at Stanford on Sept. 1.

Coach Karl Dorrell said either freshman Chris Forcier or Osaar Rasshan, who moved from quarterback to receiver this summer, will be the third-string quarterback. Rasshan has missed the last two days of practice because of lingering problems from a concussion.

Cowan, meanwhile, said he will have no trouble staying ready while his leg heals.

“I’m still in the meetings, I know what we’re doing as a team,” Cowan said. “The only difference is I’m not dropping back and throwing the ball.”

The Bruins began preparations for Stanford on Wednesday, a welcomed change on the last day of training camp.

“You really got to pay attention to what’s in front of you now,” Olson said. “You do that playing against our defense too, but there’s always that voice in the back of your mind saying, ‘You will not be playing against these guys.’ Now we can focus on the defensive scheme we’re going to see.”

With Jim Harbaugh taking over as Cardinal coach, the Bruins don’t know what to expect. Dorrell’s staff is working off films of Western Michigan, where Cardinal defensive coordinator Scott Shafer coached last season.

“It’s the first game, you never know what you’re going to see, just like they don’t know exactly what they’re going to see from us with a new offensive coordinator,” Dorrell said.

Christian Ramirez said he is making a smooth transition back to tailback after two days of practice. Dorrell approached the sophomore about switching from strong safety after Derrick Williams was forced to quit football because of concussions.

“It’s still the same instincts,” said Ramirez, who moved to safety last year. “Only thing is I’m running with the ball instead of running at it. I seem to be picking up the offense easier this time.”

Still, Ramirez said, “It felt weird carrying the ball. I haven’t done that in so long. I feel I’m running a little high right now.”

Chris Markey and Kahlil Bell will get the bulk of the carries against Stanford, but Ramirez could get time as the third tailback, depending on how much of the offense he can absorb.

“I spend a lot of time before practice and after practice with [running back] coach [Dino] Babers,” Ramirez said.

Michael Pitre, the team’s starting fullback, is out indefinitely with a bone bruise in his left knee. He had arthroscopic surgery on the knee in the off-season, but re-injured it during practice Aug. 10.

“Right now, we need to rest him,” Dorrell said. “We’ll see week-to-week as we keep working on rehab to see if we can get it calmed down.”

Trevor Theriox, who Dorrell expects to return from a hamstring injury by the end of the week, will take over as the starter.

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chris.foster@latimes.com