Bell, a junior with seven catches for 143 yards and a team-leading 20.4 yards per catch, was knocked out last week against California after taking a hard hit. It was thought at first to to be a stinger, but Mora said it might be more serious.
"He got hit pretty good," Mora said. "His shoulder is sore. His ribs and neck are sore. That side of his body where he came down hard is sore. We'll give him a couple of days to see how he is. He's feeling a little better, but he's not feeling great."
In other injury news, running back Johnathan Franklin, the Pac-12's leading rusher, was dressed and ready to go for practice even though Mora expected him to sit out because of leg and hand contusions.
"He practiced," Mora said. "I didn't think he would. He's tough."
Defensive back Marcus Rios sat out of practice because of a sinus issue, but he is expected to return Wednesday, Mora said.