The 29-year-old Wainwright, who is second in the National League in wins (20), ERA (2.42) and innings pitched (230 1/3) and third in strikeouts (213), has a strained muscle in his pitching arm.
"I strained a muscle in my upper forearm, close to my elbow area," he said. "When you do that, all those muscles are designed to help keep your elbow stable. So when you lose a muscle, then it puts more pressure on the ligaments and tendons. For me to go out there minus a muscle is probably not smart. I have a million reasons why I should or could go out there and pitch, but none of them are worth the risk."
Wainwright had an MRI done on Monday.
"It showed a strain of the muscle and some inflammation around a nerve," he said. "My ligament looks exactly the same as it did before. Everything is perfectly intact. It's really a non-issue to be honest. If we're going to the playoffs, I'm pitching."
Cardinals manager Tony La Russa said on Tuesday it was just precautionary.
"I think if we're in the race, he could pitch," La Russa said. "If we're playing in October, he could pitch. But I think it's smart. He's pitched enough. He's had a great year. It's time to pass."
Wainwright, who won 19 games last season, pitched with the issue during his last two starts. Against the Padres on Sept. 19, he allowed one earned run on five hits over eight innings with seven strikeouts. Against the Cubs at Wrigley Field on Sept. 24, he allowed one earned run on six hits over six innings and struck out seven.
"I just think the more you do it, the more risk you have of compounding the injury," Wainwright said.
The right-hander described the injury as "not a pitching one, but a sleeping one."
"I fell asleep at night sleeping with my arm under my head, and I woke up and the whole arm was completely numb -- and I know that happens to almost everybody at some point in [their] life," Wainwright said. "Usually what you do is grab with your other arm and you move it to other side, and you wait for it to tingle or whatever. It was like 3 o'clock in the morning and out of just pure laziness, all I did was just kind of fling it backwards -- and when I did, I wrenched my elbow a little bit. It's not a pitching-related injury. That's why it's not a ligament or anything like that. It's literally a sleeping injury."
Nate Latsch is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.