PHOENIX -- After the Cardinals used 10 relievers in Tuesday night’s 19-inning loss to the D-backs, they were looking for a deep start from Michael Wacha on Wednesday.

But Wacha exited in the bottom of the second inning of an eventual 9-7 loss with tightness in his right shoulder and appears to have a mild strain in the shoulder, manager Mike Shildt said. Wacha will undergo an MRI on Thursday in St. Louis for a full diagnosis and to determine the severity of the injury.

Wacha felt the tightness when warming up and thought it might loosen up during the game, but the tightness only worsened. He doesn't know what might have caused it or triggered it.

“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 allowed two runs on four hits in 1 2/3 innings, putting the Cardinals in a 2-0 deficit. He struck out three and walked two. (Mike Mayers took over and walked Abraham Almonte, a walk that was charged to Wacha.)

Wacha knew the state of the bullpen after Tuesday’s marathon and wanted to go as long as he could, but he couldn’t shake the tightness.

“That was the toughest part, actually, telling someone how I felt,” Wacha said. “You know what kind of situation we’re in with the 19-inning game yesterday, and with the bullpen, you try to give them as much of a break as you can. Just coming out of that game really killed me, but they did a heck of a job picking up the slack.”