That’s a pain in the ace.

Just when Zack Wheeler had emerged as the Mets’ must-see spring training attraction, the fireballing phenom yesterday strained a right oblique taking swings in batting practice and was scratched from his start against the Cardinals.

The Mets are calling the injury a “mild strain,” but haven’t set a timetable for the rookie to resume throwing. Wheeler joined Johan Santana among the Mets starting pitchers on the shelf.

“I came out and it was a little stiff,” Wheeler said before the Mets’ 12-4 exhibition loss to the Cardinals at Tradition Field. “It’s early in the spring. We’ve still got a month left, so I don’t want to risk a bigger injury and be out longer.”

Wheeler didn’t experience problems throwing, according to general manager Sandy Alderson, but was “sore to the touch,” leading to the Mets’ decision to scrap his start.

The 22-year-old righty wowed the Mets in his Grapefruit League debut on Saturday, when he fired two shutout innings against the Nationals and drew comparisons to his mound opponent, Stephen Strasburg, for his raw stuff.