The injury is expected to heal on its own. Petty said he feels pain and currently "can't sit down right, can't walk right," but expects to play Saturday versus Northwestern State.
The quarterback was hit from behind during his first run in Sunday's opener and was removed at halftime after playing gimpy for the rest of the first half. The injury was originally called a bruise following the game.
"I'm bummed out because it's never fun being hurt," Petty said. "When you're throwing and I can't torque [it's frustrating]. Every time I went to throw it would jolt. It felt like somebody was punching me in my side."
Petty said he is being treated with ice and other non-invasive methods during Baylor's off day on Monday. On Tuesday, he will get his first chance to test his back.
The 2013 Offensive Player of the Year, Petty finished seventh in last season's Heisman Trophy voting. He collected two touchdowns and 161 yards Sunday before leaving the game.
- Molly Geary