Foster, who was named to the Conference USA All-Freshman team last season, had started the past 17 games for the Cardinals at right tackle.
"He's not doing good," Petrino said. "We found out this morning that he has a torn knee ligament and will miss the rest of the season."
"We're going to let the swelling go down and he'll have surgery in a couple of weeks," Petrino said. "Losing him is a big loss because he was doing so well."