"We think that's where he fits best right now," manager Joe Girardi said Wednesday.
For much of spring training, Chamberlain was a contender for a spot in the starting rotation. He was a starter in college and the minors before New York switched him to a relief role last year and called him up in August to boost its shaky bullpen.
When asked if Chamberlain would go back to the setup role he thrived in as a rookie last season, Girardi refused to be specific. But he did say Chamberlain would pitch late in games.
"He's a back-end guy. That's how I'm going to classify him," Girardi said.
The 22-year-old right-hander had prepared as a starter this spring, but wasn't surprised by the decision.
"I had an idea if everybody stayed healthy this would happen," he said. "It's best for the team right now. It's about winning games. It's not about Joba Chamberlain."
The team said it still expects Chamberlain to be a starter down the road, but Girardi wouldn't commit to saying it would happen this season.
"We have not got that far," Girardi said. "I'm going to put it this way, we consider him a starter in the future. I don't want to be locked into a timetable. You know that's my personality."
Chamberlain started Saturday against Detroit in a split-squad game and allowed three runs and four hits in three innings. He has a 6.14 ERA in 7 1-3 innings this spring.
Chamberlain is scheduled to pitch out of the bullpen Thursday and Saturday. He is expected to work on consecutive days next week.
"I really wasn't wondering at all," Chamberlain said. "It's great not only to know, but just to get your routine going. The routine is going to be the hardest part. Sitting there [in the bullpen] and when the inning comes up, get ready."
Chamberlain went 2-0 with a 0.38 ERA and one save in 19 games last season. He struck out 34 and walked six in 24 innings.