Since coming back from a slight case of shoulder stiffness on June 7, Herrera had pitched in 20 games with a 0.46 ERA and held opponents to a .191 average through his scoreless seventh inning on Wednesday night.
The team's record in those 20 games was 14-6.
"He's commanding the ball good, he threw a couple 100-mile-an-hour fastballs, his changeup's been good, he's developing his breaking ball that he's using more consistently. He didn't have the curveball last year, he was strictly fastball, changeup," manager Ned Yost said.
"Last year, he was wild in the zone -- a lot of fastballs right down the middle. But he's keeping the ball [down] better, working the corners better, just normal stuff to make you successful."
Last year, Herrera was shipped off to Triple-A Omaha a couple of times but there's been no Minor League banishment for him this year.
"You never want to be in Triple-A. It was the first time in my career so I was a little sad. But I said, 'All right, let's get going,'" Herrera said.
Determined to improve himself, Herrera used the offseason to study how-to videos that he's found of prominent pitchers of the past.
"I watch a lot of video on how these guys pitch -- Pedro Martinez, Greg Maddux, Roger Clemens. You've got to learn from the best people," Herrera said. "It's fun to watch those guys pitching. The way they explain their pitches is awesome."
The self-teaching obviously helped.
"He's learned to become a better pitcher," Yost said.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.