Backups Sullivan and Mustain have thrown a combined total of only 17 passes for their respective teams this season, but injuries to starters Rudy Carpenter and Mark Sanchez have put their status in doubt.
Carpenter sprained his left ankle during ASU's 24-14 loss at California last weekend, and left Memorial Stadium on crutches. Sanchez suffered a bone bruise in his left knee in the third quarter of USC's 44-10 victory over Oregon.
Carpenter has started 36 straight games for ASU -- the second-longest active starting streak by a quarterback in the country.
"He's still in a boot," Sun Devils coach Dennis Erickson said Tuesday. "It's a day-by-day thing with him. He won't practice today for sure. We'll just see how he is on Wednesday and Thursday. He has to be pretty close to 100 percent."
Sullivan, a junior, has completed 1-of-8 passes for minus-1 yard this season and hit on 26-of-52 for 211 yards in his career.
"This is what he has been working for," Erickson said. "To go down and have your first start in a college game against USC is very challenging. There is no question about that. But I really like him. He is very competitive and knows what we're doing. So if that's the case, he will do a good job."
A decision might not come before game-time.
Sanchez, in his first full season as USC's starter, has already been selected the Pac-10 offensive player of the week three times this year, having thrown for 1,069 yards and 13 touchdowns.
"It's day-to-day with Mark," USC coach Pete Carroll said. "We'll wait and see how he's going to do. We're very fortunate, and he's very fortunate in this injury, that he didn't have any damage other than the bruise. But it's still very sore and stiff.
"He'll come back as soon as he's able to get full flexion, and all that, run around on it. That may be tomorrow, it may not be. I don't know. I'm just going to wait. If he's not able to go full speed but still take some snaps, do some walkthroughs and things like that, it will weigh into all that."
Sanchez was injured when sacked by former high school teammate Nick Reed. Mustain, a sophomore from Arkansas who was promoted from No. 4 to No. 2 on the depth chart last week, completed 5-of-8 passes for 111 yards, with a touchdown and an interception against Oregon.
"I know he's really pumped up about the opportunity," Carroll said of Mustain. "He did a nice job last week, which helps him kind of lead into this week."
"He had a particular day last week that he just looked like he really was in command of stuff," Carroll said. "He looked like the best guy. But you couple that with the competition and Aaron Corp and Garrett Green had chances to do as well and didn't show as well. So it was kind of a combination of things that happened."
Before the season, it appeared the ASU-USC matchup could be the game of the year in the Pac-10, since the teams finished tied atop the conference last year (the Trojans beat the Sun Devils to win the tiebreaker) and were picked to finish 1-2 this year.
But Arizona State (2-3, 1-1 Pac-10) has lost three straight, and USC (3-1, 1-1) was upset 27-21 at Oregon State on Sept. 25. Arizona and California are currently tied for first place in the conference with 2-0 records.
"They look the same [as last year]," Carroll said of Arizona State. "They have a great offense. Rudy is all over the place, throwing the ball everywhere. He's just a really, really good player. He makes things happen."
Carroll said he didn't know much about Sullivan, adding that USC assistant Nick Holt recruited him a few years back.
"He's got a real strong arm, big kid, kind of a classic pocket-type of quarterback," Carroll said.
Carroll said he was pleased the Trojans bounced back so well against Oregon after the loss at Oregon State.
"We didn't play the kind of football we're capable of [at Oregon State]," he said. "It's amazing. That line is so fine. I really believe in the Oregon State game had we made a couple things happen early in the game, everything would have been fine."