Off Prior, in the righthanded's simulated game yesterday.
From the official site: "He was sharp, he's got good stuff," said Macias, who did hit a home run off Prior. "I didn't think the fastball would be that hard, but it jumped. And the off-speed, the curveball, wow. It was good. He'll be back soon." Prior may skip the minors' assignment and rejoin the rotation in late June/early July.