Harvey, who will be available coming out of the bullpen starting Tuesday, was not happy about the decision.
"It's the decision that they made. I have to suck it up and go out there and do everything I can to get things back in gear," he said. "I don't have to agree with it, but I have to go out there and do the best I can to get things going and do everything I can to help this team get back to where I believe I can help this team, and that's as a starter.
"On a scale of 1 to 10, obviously I'm at a 10 with being pissed off, but my performance hasn't been there, and I just have to do whatever I have to do to get back in the starting rotation. And that's right now go to the bullpen and work on some things, get some things back to where they need to be and get my s--- in order and figure it out."
"He threw a bullpen today, and he'll be available Tuesday,'' Callaway said. "I don't think he is obviously very happy about it. I wouldn't say he's discouraged. I think he's motivated to go out there and show everybody that he can be a starter again at some point.
"He can help us ... it's inevitable that he's going to make more starts for us this year. That's how baseball goes. Everybody uses tons of starters; 11 was the average last year for every team."
Since a solid first start, Harvey has struggled, most recently when he allowed six earned runs in the first three innings Thursday night in Atlanta as the Braves beat New York 12-4. That dropped Harvey to 0-2 with a 6.00 ERA. In 26 innings over four starts, he has allowed 26 hits, 4 home runs and 4 walks.
With the return of injured left-handed starter Jason Vargas imminent, the Mets have been considering going to a six-man rotation or moving a current starter to the bullpen.
Instead, lefty Zack Wheeler (1-1, 2.77) will remain in the rotation with Noah Syndergaard, Jacob deGrom and Steven Matz. Vargas, recovering from a broken bone on his non-pitching hand, will make a rehab start Monday for Triple-A Las Vegas and then join the rotation.
Harvey, 29, has allowed a combined 14 runs in his last three starts. He was 5-7 with a 6.70 ERA in 2017.
The right-hander was an All-Star in 2013, but missed the 2014 season following Tommy John surgery. He helped the Mets reach the World Series in 2015, missed half of the 2016 season following surgery to correct thoracic outlet syndrome, and he was out more than two months last season with a right shoulder injury.
Harvey has started 104 major league games and come out of the bullpen just once, doing it last season.
The Associated Press contributed to this report.