Sanchez jumped up to make a pass during the team's agility drills, landed awkwardly on his foot and twisted his knee.
"He was real scared that he had hurt himself," Pete Carroll said.
After practice Coach Carroll met with team doctors and was relieved to learn that Sanchez's injury wasn't considered serious. The redshirt junior suffered a dislocated left kneecap but didn't have any complications and is considered day-to-day.
Sanchez will begin rehabilitation on Monday.
"Mark dislocated his kneecap. They put it back in (on the field). There are no other complications. It's a day-to-day deal. We'll wait and see. He's real encouraged and the doctors are, too. It will be immobilized over the weekend, then he'll start rehab on Monday. It's day-to-day and we'll see how it responds.
"On the other side of this, it's an extraordinary opportunity for the other quarterbacks to jump in and fight for the job.
"Before practice, Mark was throwing and came down on the side of his foot. We'll have to wait and see. As is always the case in situations like these, this is an extraordinary opportunity for the other quarterbacks. We're so lucky to have guys like Mitch Mustain and Aaron Corp to jump in until Mark gets back.
"How we handle it and how we deal with it is what's important. Those guys jumped in today at practice and took the team down the field. Both of them are championship kids.
"The concern we have now is that we only have 2 quarterbacks. Who's going to be number 3 as insurance is the question. Somebody like (quarterback-turned-wideout) Garrett Green might get some work there now.
"Mark's encouraged, he's smiling and he knows he's going to be alright."
"It's a dislocated kneecap. I landed awkwardly when throwing. It's pretty clean. Nothing torn and no cartilage problems. It's the best case scenario. I'll start rehabbing on Monday and be back as soon as possible.
"I feel good about the other guys being able to fill in. It'll be a seamless transition. They'll do well.
"It was scary, frightening at first. The way they say these things happen, it could be a lot worse. It just hurt a lot when it happened, but it feels better now. It's day-to-day starting Monday. I have heard about people who come back from this in a short period of time and others in a long period. I'm hoping mine is short.
"I'm hoping to be back for the opener. I'll work to get back before that. I'll get back the fastest way possible.
"As soon as they said it wasn't an ACL or MCL, it was a relief. It's a little frustrating to have this happen, but it'll make me work hard."
Quotes courtesy of USC Sports Information