The scare came during Game 1 of the AL Division Series against the Angels in Anaheim. Herrera walked David Freese on five pitches and there was an immediate medical conference on the mound. Herrera exited.
"The [right] forearm was real tight and two fingers were numb. You can't feel the ball and what do you do?" Herrera said after Tuesday's workout at Kauffman Stadium.
"As soon as I got a massage, it was better. So now I'm good. Ready to pitch against Baltimore."
Starting pitcher James Shields was so concerned about Herrera's departure last Thursday night that he followed him back to the clubhouse.
"When he came out of the game, I actually met him up in the training room just to see how he was doing," Shields said. "I've seen a lot of injuries over the years, and the way he explained it to me, I really wasn't too concerned about it."
Herrera, though, was very worried as he headed toward an MRI the next day.
"Every time you have something like that, you think it could be Tommy John [surgery]. That was my biggest concern," Herrera said.
"That's why I was nervous about the results. Once they said I was OK I was, 'Wooooooo, I'm back!' I knew it was just tightness, but you never know. It might have been tendon damage. Malo."
Herrera wasn't so sure about manager Ned Yost's theory that he just experienced a cramp caused by the high temperatures at Anaheim. He doubted that a cramp would cause his fingers to go numb. Anyway, he's over it.
"Now I feel normal," he said.
After skipping Game 2 at Anaheim, Herrera returned to pitch a perfect seventh inning in the ALDS clincher at Kauffman Stadium. He hit 101 mph on the radar gun.
"Anytime you can come back strong like that, it's a good sign," said fellow reliever Wade Davis. "If he'd have gone out there and not been himself, that would've been scary, but he came back pretty good."
Herrera is the seventh-inning link to Davis in the eighth inning and Davis is the link to Greg Holland in the ninth.
As Yost puts it, that makes it a six-inning game. Get a lead to those three and it seems like a lock.
"You look at their numbers and it's the best I've ever seen for a back end three guys, and we owe a lot of our success to them," Shields said.
Combine the three relievers' statistics for the regular season and this is some of what you get: 14-8 record, 49 saves, 1.28 ERA, 204 1/3 innings, 258 strikeouts, 69 walks, three home runs allowed.
"We're the three luckiest guys?" Herrera said, laughing. "Yeah, we're lucky. Maybe."
Or, maybe, just very talented. Hard working, too. Davis pitched in 71 games, Herrera in 70 and Holland in 65.
"You always had a setup man and closer," Herrera said. "But when you've got a third guy, you're good to go."
This is the first postseason experience for Herrera and, now that his little rendezvous with an MRI machine is past, he's having a great time. He playfully pointed out that he's pitched in October before.
"I did in 2012. The season ended on Oct. 3," Herrera said, "but it wasn't the playoffs. This is unbelievable -- how you feel the adrenaline, the big crowds. It's so nice. From now on I want to be here every year. Every October. Bueno."
Herrera is right where he needs to be this October, in the Royals' bullpen waiting for the seventh inning.
"It's definitely a relief to have him back," Shields said. "He's a very important and vital key to our success, so to have him be healthy and feeling good is awesome."