Herrera, who has given up eight home runs in 20 1/3 innings (he surrendered only four all last season), certainly qualified for a demotion. He already has lost four games and sports a rather unsightly 4.87 ERA.
But, to be fair, Herrera wasn't struggling any more than Moustakas, who is mired in a miserable slump – 2 for 32 on the road trip – and is hitting just .176 this season.
Moustakas was back in the lineup Thursday night against the Angels, while right-hander Luis Coleman was recalled to replace Herrera in the bullpen.
Royals general manager Dayton Moore told FOXSportsKansasCity.com that the Royals have not considered shipping out Moustakas.
"It's our philosophy that once a hitter reaches the major leagues," Moore said, "he's going to develop much faster at this level than he would if he goes back to Triple-A.
"Hitters need to establish a catalog of pitchers up here so they get used to the league and get comfortable. It's not helping his development by sending him down."
Optioning a hitter such as Moustakas is not out of the question, however, Moore said. The Royals did so with Alex Gordon and Billy Butler when they were younger.
"But really, it comes down to three things," Moore said. "Has the player lost faith in himself and does he doubt himself? Has the manager's faith in that player wavered? And has his teammates' faith in him lessened?
"I think we can fairly say in all three cases with Moose that none are true. Moose is battling through a tough stretch in his career but he has stayed positive, and the manager has stayed positive about him, and his teammates are completely supportive of him. It's not a good idea right now to move Moose."
With Herrera, though, the Royals have seen a pitcher who has lost confidence in his command. Herrera also recently lost his job as closer Greg Holland's primary set-up man.
"What we saw in the Angels series was a guy who was trying to alter his delivery just to make a pitch move a certain way and get it over for a strike," Moore said. "Those are the types of things that can put a pitcher's health in jeopardy. So we thought it was best for him to go back to Triple-A and work through the mechanics.
"We can control his outings in Omaha much more than we can here."
And Moore said it's harder for pitchers to work through slumps at the big-league level.
"With a hitter, he can sometimes hide in a lineup," Moore said. "With a pitcher, when he's in the game, he is completely exposed. It's just him, and if he's struggling, it really affects the whole team more so."
Moore said he informed Herrera, 23, he was going to be sent down Thursday morning.
"He took it well, like a pro," Moore said. "He'll be back up. He's too talented. This is just his second full season. He's a young guy and he's scuffling a bit right now, but he'll figure it out and be back even stronger."
Coleman, who pitched in 42 games in relief for the Royals last season, was sensational so far this season for Omaha – a 2-1 record with three saves and a 1.04 ERA.
You can follow Jeffrey Flanagan on Twitter @jflanagankc or email him at firstname.lastname@example.org.