Rookie right-hander Joe Musgrove will take McCullers' rotation spot beginning with Sunday's start against the Rangers. The Astros called up right-hander Brad Peacock from Triple-A Fresno to take McCullers' spot on the roster; he'll pitch in relief.
As of now, surgery doesn't appear to be an option for McCullers.
"I never want to be off the mound," McCullers said. "I always want to be out there. I would pitch every day if I could. So it's tough to deal with that from that standpoint, but all we can do is follow the doctor's orders, and we have. That's what we're going to do."
Considering McCullers won't pick up a baseball for two weeks, he's likely out for the rest of the month. He'll have to begin a throwing program, which will include bullpen sessions, a simulated game and a Minor League rehab stint before being activated. It's going to be a long road.
"The diagnosis has been a sprain, and we'll give him a couple of weeks to cool down," manager A.J. Hinch said. "He's got some inflammation. The muscles around his elbow are pretty tight today because he's trying to protect his body. That's what happens when guys throw like they do. We'll know more in the next couple of weeks, but what I can tell you is he's not going to do any throwing for two weeks. We'll re-evaluate him, and hopefully he'll be feeling better and we can build him back him up."
McCullers started the season on the disabled list with soreness in his shoulder and didn't make his first start until May 13. He posted a 2.08 ERA in five July starts, with 44 strikeouts in 30 1/3 innings. That's including back-to-back 10-strikeout games on July 22 and July 27. He was pulled from Tuesday's game in the top of the fifth when Hinch noticed he was uncomfortable.
"I felt the best I felt all year yesterday, which is the [coincidental] part for me," McCullers said. "It's really weird for me to feel how I felt yesterday before coming out and then have to sit here today. We're just going to take it step by step, but I'm looking forward to whenever that comes to get back there."
Peacock, who appeared in 47 games with Houston in the previous three years, was 5-5 with a 4.13 ERA in 19 games at Fresno, including one complete game. He'll be used in the long relief role previously shared by Musgrove, Scott Feldman and Chris Devenski. Feldman has been traded, and Devenski was unavailable on Wednesday following his long outing on Monday.
Brian McTaggart has covered the Astros since 2004, and for MLB.com since 2009. Follow @brianmctaggart on Twitter and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.