Chapman, entering his third season with Cincinnati, had hoped to get back into a starting role after spending most of the last two seasons pitching in relief. Manager Dusty Baker said injuries forced the change in plans on Monday.
"That was a very, very tough decision because Chapman could be one of our best starters or best relievers," Baker said. "It is a situation where with the injuries that we have, starting out early in the season when the starters are lucky to go five or six innings, we had to shore up our bullpen."
Closer Ryan Madson is out for the season after tearing a ligament in his elbow. Setup man Nick Masset will start the season on the 15-day disabled list with an inflamed pitching shoulder. Left-hander Bill Bray missed most of spring training with a strained groin and is still getting sharp.
The rotation is set, so Chapman would have opened the season in Triple-A if he was a starter. The Reds decided to keep him in the majors as a reliever for now.
"This early in the year, the bullpen does a third to half the work," Baker said. "It only takes one dude to have one bad inning. One dude can have a bad inning and ruin hours of work."
Baker also announced Monday that Sean Marshall will open the year as the Reds' closer in place of Madson.
"Sean Marshall's our closer for now," Baker said of the lefty, who went 1-0 with a 3.27 ERA in 11 innings over 10 Cactus League outings.
"He's closed a few games, not a lot," Baker said. "He can throw that breaking ball for strikes anytime he gets ready and that's hard to do in Arizona with this light air and all. He's not fazed by situations. He's been around awhile. He's a veteran guy and has had success."
The Reds signed Chapman to a six-year, $30.25 million deal before the 2010 season, expecting him to develop into a hard-throwing starter. With their rotation set, they moved him into a relief role midway through that season, then called him up in August. He went 2-2 with a 2.03 ERA in 15 relief appearances as the Reds won the NL Central.
Chapman struggled with his control last season and spent time in the minors. He was 4-1 with a 3.60 ERA in 54 appearances for Cincinnati overall.
The Reds expected him to open this season in Triple-A as a starter. He made five appearances as a starter in spring training, going five innings in each of his last two outings. He knew there was a chance he could end up in the bullpen.
"Chapman is going to be a starter," Baker said. "He is going to be a very good starter. He's a smart kid. He actually anticipated it. When we lost Madson and Masset and Billy Bray is not sharp yet, through the process of elimination, he figured that stuff out."
The Reds will carry 11 pitchers until Saturday or Sunday. They open the season on Thursday against Miami, then have a day off, allowing them to carry one less pitcher and one more position player through the first weekend.
Also Monday, the Reds optioned shortstop Paul Janish to Triple-A Louisville, leaving 25 players in camp.
Information from The Associated Press was used in this report.