In the eighth inning, Herrera came in to counter right-handed hitter Mike Napoli but gave up a four-pitch walk and then a three-run homer to Daniel Nava that resulted in a 4-3 Red Sox victory.
After the Atlanta incident, there was speculation that Herrera might be tipping his pitches.
"He's not tipping his pitches. It's poor location," manager Ned Yost said. "Look, it's as bad walking Napoli on four pitches as it is giving up a three-run homer. It's location on his pitches. His stuff is fine. The key matchup of the inning was Kelvin against Napoli, but that came out of play with a four-pitch walk."
Yost said he hasn't lost faith in the young right-hander, who didn't give up a run in six appearances before Atlanta.
"I don't bail on guys after a bad outing or two bad outings," Yost said. "He'll get on a good roll and be fine."
Herrera dismissed the idea that he was tipping his pitches.
"No, that's just an excuse," he said.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.