Trevor Rosenthal, moved back to the bullpen, fired four pitches clocked at 100 mph as he fanned the side in his only inning, the seventh during the Cardinals spring training game Sunday against the Mets.

He recently had been told he was going to open this season as a reliever rather than a starter.

Rosenthal said he heard the mild gasps from the stands when he threw 100.

“It’s definitely fun to ... have that special ability,” he said. “Ultimately, it’s not the most important thing, so I try not to focus too much on that.”

Matheny thought Rosenthal was a “different pitcher” Sunday when he knew he had only one inning rather than trying to navigate more innings as a starter.

“He’s an absolute weapon to be used out of the ’pen,” Matheny said.

“I think we’re better with him out there. The way he’s throwing makes him extremely tough to face for an inning.

“He came in here trying to compete for a (starting) job and when it didn’t happen, I’m sure it was a little hit to him. But he started saying the right things and immediately got back on the mound and proved it.”