Greenard, the team's best pass-rusher, got hurt early in the first quarter and was seen on the sideline in a full cast. Petrino said Greenard has a ligament injury and will be out "a while."
"It's a shame, because he worked extremely hard to prepare and get ready for the season," Petrino said at his weekly news conference. "But he landed on it really differently and injured it."
Greenard, a team captain, led the Cardinals with 15.5 tackles for loss and tied for the team lead with seven sacks a season ago.