"Any time we have to take him out, you know it's pretty bad," manager Mike Matheny said. "But we'll know more tomorrow. He went through some tests, and you could feel some irritation there, obviously. He got it taped up and was convinced he was going back in, but we needed to get it looked at."
Molina initially remained in the game after Matheny and a team trainer evaluated him on the basepaths. He manned his position in the top of the third inning before being removed for a pinch-hitter -- backup catcher Tony Cruz -- in the bottom half of the frame.
Molina, in line to start the All-Star Game on Tuesday, is hitting .287 this season and .357 this month. He has been a beacon of consistency behind the plate, appearing in 83 of 92 games this season and starting the last 14.
"His left thumb, I'm telling you right now, has been in bad shape for a long time, as most catchers' thumbs are," Matheny said. "You get those foul tips, or you get a ball that goes the wrong way, it messes with that ligament a little bit in the thumb. He's had that a little while. It's painful. But to have it on the right hand, I've never had that. Once again, we'll have a whole lot more information when we let the doctors get their results."
If the injury sidelines Molina for an extended period of time, he will miss starting a second straight All-Star Game. But more important, it would jeopardize his ability to play this weekend against the first-place Brewers, whom the Cardinals now trail by only two games following Milwaukee's loss to Philadelphia.
Cruz will assume Molina's duties in his absence. The fourth-year catcher has appeared in 20 games this season, starting 11, and hitting .250. Utility man Daniel Descalso is the team's emergency backstop, though the club could call up a reinforcement if needed.
Alex Halsted is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.