That will be Saturday against the World Series champion Giants. So far he's pitched two scoreless innings, giving up three hits.
"I need to add a pitch this season," Herrera said. "It's a sweep curveball."
Manager Ned Yost said an effective curveball will make Herrera even more dominating than he was last year, when he posted a 2.35 ERA, four wins and three saves as a rookie.
"It's definitely the one thing that he needs to make a bit of an improvement on and be more consistent with," Yost said. "He basically throws strikes with his fastball, has a very good changeup and was a tad bit inconsistent with his breaking ball. If he can develop more consistency with his breaking ball, that gives him three above-average pitches that he can get guys out with."
This spring, Herrera has a secure hold on a bullpen spot. A year ago, he was a kid hoping to land a job.
"Last year, I was worried about where I was going to be," Herrera said.