Wainwright is 20-11 with a 2.42 ERA. He says he hurt his elbow while sleeping on it earlier this month, and was bothered by pain while winning his last two starts.
"You know how you sleep with your arm under your head?" Wainwright said Monday. "I did that, but instead of grabbing my hand and moving it to the side, I just flung it. I kind of wrenched it."
Wainwright was set to start Thursday against Colorado. He will be examined in the next day, and manager Tony La Russa said a decision will be made at that time.