Manager Mike Shildt said more would be known Thursday, but that the injury appears to be a mild strain. Wacha will undergo an MRI on Thursday.
Wacha had a 2-0 count on Diamondbacks right fielder Abraham Almonte in the bottom of the second inning when a trainer came out to check on him. After a short conversation on the mound, Wacha and the trainer walked into the Cardinals' dugout.
"We evaluated him in between innings, and it was coming out all right," Shildt said. "He threw some [93-94 mph fastballs] up there, but the inning started to go a little bit. We're always going to err on the side of caution with our guys, so we went out and checked on him, [he] said it was a little tight. At that point, [we're] just not going to push it. Too much respect for him and his career and health."
Wacha was charged with two runs on four hits in 1⅔ innings. He made his 29th appearance and 24th start of the season, striking out three and walking two on 43 pitches.
Wacha, 28, hasn't won a game since July 19. His three strikeouts gave him 759 in his career and tied him with Mort Cooper (1938-45) for 19th in Cardinals history.
Information from The Associated Press was used in this report.