ARLINGTON -- The Yankees have invested a good chunk of their time and hinged a large part of their future on Joba Chamberlain's health. It began last year with his bullpen debut and the long list of "Joba Rules" that went along with it. It continued this season with Chamberlain's carefully mapped transition to the big league rotation. And on Monday, it fell apart when he suffered an injury, anyway.

Chamberlain left Monday's start vs. the Rangers after 4 2/3 innings with tightness in his right shoulder, and almost certainly will miss his next start. Though the actual severity of his injury is not yet known, Chamberlain will fly to New York early Tuesday morning for testing.

"He's the type of guy that can anchor your staff and that can be your ace when postseason time comes around," outfielder Johnny Damon said after New York's 9-5 loss. "We'll have to wait and see what the tests show, but with a young kid like that, you can't take risks."

The Yankees didn't, removing him from Monday's game in the fifth inning at the first sign of trouble. Chamberlain said he didn't feel anything abnormal in his arm heading into the start, nor did he feel anything throughout the first four innings. But he felt a "grab" in the deltoid area of his right arm during the fifth inning, and New York's coaching staff noticed him trying to shake the pain out of his arm.

Manager Joe Girardi, trainer Gene Monahan and several players gathered around Chamberlain on the mound for a brief discussion. Moments later, Chamberlain walked to the dugout, clutching his right shoulder with his left hand.

"As a pitcher, unless my arm falls off, I'm probably not going to say anything," Chamberlain said. "That's just the way it is. Sometimes you've got to learn that maybe you should call somebody, but it's something that it grabbed and popped and got stiff."

Chamberlain said that he remained unconcerned for two reasons: He never had shoulder issues before Monday and both his strength and velocity were fine. The Yankees confirmed that much with a series of resistance tests back in the clubhouse Monday, and they will confirm whether his injury is muscular when team physician Dr. Stuart Hershon administers an MRI -- among other tests -- at New York's Presbyterian Hospital on Tuesday.

Chamberlain said that the injury did not affect his performance on the mound -- he allowed five runs on eight hits -- and that he was unsure if he would miss any time. But Girardi admitted that his pitcher likely will miss at least one start.

"We don't think it's anything serious," Girardi said. "I'm not really concerned that he's going to be shut down for a while, but there's a chance that he's going miss a start, and we might have to be a little bit creative."

When Chamberlain left the game, immediate speculation focused on a play in the fifth inning, when Chamberlain fell to the ground to avoid the ball. With a runner on first base and no outs, Rangers second baseman Ian Kinsler hit a slow dribbler out in front of home plate that umpires initially ruled fair. Catcher Ivan Rodriguez pounced on the ball and fired to second base, forcing Chamberlain to crumple to the ground.

But Chamberlain insisted that the play had nothing to do with his shoulder.

"I thought he called a foul ball so I kind of stood there, and Pudge threw a bullet right over my face," Chamberlain said. "I had to get out of the way."

And now he'll be miles away, back in New York for the indefinite future. Assuming Chamberlain misses at least one start -- and the Yankees are operating under that assumption -- they will need to find a starter for Saturday's game in Anaheim. One option is Ian Kennedy, who last pitched Sunday in Pawtucket, R.I., and could throw Saturday on an extra day of rest. But that would require a roster move to clear space for Kennedy, creating the possibility that they could choose to start both Dan Giese and Darrell Rasner this weekend.

The team announced earlier Monday that it had promoted Giese to the fifth starter's role in place of Rasner, but both pitchers would be available this weekend if needed.

"You wait for the tests, and you see what that reveals," Girardi said. "And then you deal with the situation."

Though Chamberlain does have a history of injuries, including a triceps strain during his final college season and a hamstring strain that hampered him for the first two months of last year, he never experienced shoulder trouble until Monday.

Making this injury more vexing is the fact that the Yankees have treated Chamberlain's health with extreme caution ever since his introduction to the Major Leagues. When Chamberlain first joined the team in a relief role last August, the Yankees created a series of rules that limited when and how much their young star could pitch. Slowly, the club loosened the reins, but Chamberlain remained limited for the balance of the season.

Spring dawned with controversy over whether Chamberlain would remain in the bullpen or become a starter, which had been his role in the Minor Leagues and at the University of Nebraska. When the Yankees ultimately decided on the latter, they spent an entire month conditioning him with increasing pitch counts.

Chamberlain's health -- even more than his immediate success -- was paramount.

"In a sense, you can put them in situations where they're not going to get hurt," Girardi said. "But sometimes, there are things you can't prevent. Some things are going to happen."

This marks the second straight season the Yankees have seen one of their top young pitchers injured at the Ballpark in Arlington. In his second career start last May, Phil Hughes pulled his left hamstring after pitching 6 1/3 innings of no-hit ball.

Hughes didn't return from that injury for three months, though Chamberlain has reason to believe -- and an inclination to hope -- that his injury won't last quite so long.

"Life's too short to worry," Chamberlain said. "You hope everything will be OK and you think it will be, and you just go from there."