Coleman had not been especially effective in six outings before Friday night, when he turned in 1 2/3 scoreless innings late in a 6-2 loss to the Yankees.
"Coleman has found something in his delivery, and last night was the best he's thrown since he's been here," manager Ned Yost said. "But he's more of a situational right-handed pitcher. By that, I mean we like him when they bunch up righties. His command has got to be better to face lefties, and right now, it's not polished enough to get lefties out."
Among the other right-handers in the bullpen, Yost wanted to keep Nate Adcock and Luis Mendoza as long men, and Jonathan Broxton is the closer.
"Right now, [Kelvin] Herrera is very valuable to us, because he can get lefties out with this changeup and he throws 100 mph. [Aaron] Crow, of course, is our All-Star," Yost added.
Coleman returned to Omaha with a 4.15 ERA in seven games. He gave up eight hits, four walks and four runs in 8 2/3 innings.