Wainwright had a 2-2 count on Darin Erstad with nobody out when he signaled to the dugout that his finger was hurt. St. Louis manager Tony La Russa and a trainer came out to the mound and La Russa brought in Kyle McClellan to relieve.
Wainwright, 5-3 with a 3.12 ERA coming into the game, allowed seven hits and two runs with four strikeouts. He had pitched at least 5 2/3 innings in 13 consecutive starts.
The Cardinals can hardly afford to lose another pitcher.
Todd Wellemeyer flew back to St. Louis after Thursday's start in Washington with inflammation in his right elbow and several other pitchers are rehabbing, including Chris Carpenter (elbow) and Jason Isringhausen (hand).
Mark Mulder, who's been out all season with a left shoulder injury, threw a bullpen session before Saturday's game. La Russa gave no timetable on his return.