To make room, the club designated outfielder Justin Maxwell for assignment.
The Royals wasted no time getting Coleman into action. He pitched a perfect ninth inning in Friday night's 4-0 loss to the Orioles.
Coleman, 26, is a part of a clan that has the Major Leagues' only family with three generations of pitchers. His grandfather, Joe Coleman, was an American League All-Star in 1948 and pitched in the 1940s and '50s, mostly with the Philadelphia A's. And his father, Joe Coleman, was a 1972 All-Star with Detroit and pitched from 1965-79.
The current pitcher's full name is Joseph Casey Coleman, but he goes by "Casey." And according to the Royals, no one by that name has ever played for the club.
Coleman relieved in eight games for Omaha, with a 1-0 record and a 4.15 ERA. He came from the Cubs organization and made 48 appearances, including 26 starts, for Chicago from 2010-12, going 7-13 with a 5.76 ERA.
"They made me a full-time reliever last year and since then, I've pretty much been a guy that can pretty much do anything -- one, two, three innings; mop-up guy; late-inning guy; extra innings, anything," Coleman said.
Coleman spent all of 2013 with Triple-A Iowa, going 5-3 with a 3.16 ERA in 48 games (four starts).
"He's a guy that's got really good stuff: 95-mile-an-hour fastball, good breaking ball, good competitor," manager Ned Yost said. "Grew up in a baseball family, been throwing the ball well down in Omaha."
Maxwell was used sparingly by the Royals this year, appearing in 16 games with a .138 (4-for-29) average. He was obtained last July 31 from the Astros in a Trade Deadline deal for pitcher Kyle Smith.
Maxwell hit .268 with five homers and 17 RBIs in 35 games for the Royals in 2013.
"It was really tough to find him at-bats. When you've got five outfielders and three of them are your starting guys that play every day, it's hard to find at-bats for certain guys," Yost said. "He's the kind of guy that needs consistent at-bats to be productive. He was a great part of this team, great in the clubhouse, and we're going to miss him."
For the last week, the Royals had been operating with an 11-man pitching staff instead of the usual 12.
Yost was asked how the organization decided to make the move back to 12.
"Did you see the game last night? Did anything strike you funny?" Yost said, referring to Thursday night's 2-1 loss to the Orioles. "If we tied that game in the eighth inning, I had one pitcher left. I got into a spot where I had [closer Greg] Holland ... so we had to do something, we had to get that 12th pitcher up here. We held on as long as we could."
Yost used four relievers and he still had Holland, who could go an inning or maybe two. But the other remaining possibilities were Wade Davis, who had a sore neck, and Danny Duffy, who is scheduled to start Saturday's game. If Yost had used Duffy, that probably would have meant Duffy going to Omaha with another pitcher coming to start on Saturday.
Having his grandfather and father as Major League pitchers made it easier for Coleman to adjust to pro ball. As a youngster, he got to inhabit big league clubhouses.
"It was unbelievable. My dad was pitching coach for the Angels and Cardinals when I was young," Coleman said. "Just being in the clubhouse and with the Angels, I remember Jim Edmonds, Troy Glaus, Garret Anderson -- they were a great group of guys and they were my idols growing up. Just being able to be in the clubhouse around them was a dream come true and I was very fortunate."
Yost is going to have to be careful when he calls for Coleman to warm up. There's also Louis Coleman, another right-hander, in the Royals' bullpen.
The newest Coleman was reminded that the Royals had his first name on the front of their batting practice jerseys -- you know, "KC."
"That is kind of funny," Casey said, glancing down at the letters. "I didn't even think of that, actually, until you said that."
Dick Kaegel is a reporter for MLB.com. Jackson Alexander is an associate reporter for MLB.com.