From Ed Price:

Rivera, who has established himself as a Hall of Fame closer with only a cut fastball and occasional sinker, also toyed with a change-up.

"Nasty," Guidry called it.

Rivera said he was "just messing around." Asked if he might incorporate a change-up, he said, "We'll see."

Jim Baumbach mentions that Mo playing around with a change up in Spring Training is a yearly deal that has never come to fruition before.