"He felt something grab or bother him on that throw that got away from him," Cardinals manager Mike Shildt said. "We'll have a better idea in 24 hours."
In the bottom of the second inning, O'Neill tried to make a throw home on Freddy Peralta's RBI single. The throw, which clocked in at 96.7 mph, according to Statcast, sailed far past catcher Yadier Molina to the backstop.
O'Neill was making his second straight start in place of Harrison Bader, who has been sidelined with soreness in his right hamstring. Drew Robinson, recalled earlier in the day from Triple-A Memphis, came on to pinch-hit for O'Neill in the third inning.
"The good thing is we don't think there's anything wrong structurally," Shildt said.
“The pitch before the home run, I felt kind of like a cramp," Mayers said. "I didn’t think anything of it. It didn’t get any better."
Mayers will return to St. Louis on Tuesday for further examination, but Shildt said he expected Mayers to wind up on the injured list.
"He said he felt something kind of cramp up under his arm pit ... and clearly, the ball wasn't coming out as good toward the end of the at-bat with [Travis] Shaw," Shildt said "He'll probably be on the [injured list]."
Mayers has a 5.40 ERA in eight appearances this season with eight strikeouts and five walks in 8 1/3 innings of work.