SARASOTA, Fla. -- The contingent of critics who believe Joba Chamberlain would help the Yankees more in the bullpen has lost one of its loudest voices. Jorge Posada has left the club.

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The veteran catcher made headlines last September by sharply stating that the 23-year-old right-hander would not hold up as a starting pitcher. Having watched Chamberlain's progression this spring in New York's rotation, Posada has reconsidered his stance.

"There's no question -- he's our fifth starter and I'm happy with that," Posada said. "I look for the betterment of Joba here. Give him credit, because he was so good out of the 'pen. I hadn't seen him as a starter, so he's been showing me and proving me wrong."

Chamberlain completed his spring workload on Tuesday, hurling 5 1/3 innings of two-run, five-hit ball against the Reds at Ed Smith Stadium. Chamberlain walked three and struck out six in the outing, tossing 53 strikes among his 87 pitches.

"I've still got to be better, but as far as velocity and just attacking the zone, I think it was the best [start]," Chamberlain said. "I felt good going through. It's got to be better, that's for sure, but it was something to build on."

Preparing as a starting pitcher for the first time in big league camp, Chamberlain's spring did not begin with such high notes. He allowed two runs in his first inning on Feb. 28 against the Twins, and Team Canada roughed him up for five runs in a March 3 start where he didn't record an out.

But Chamberlain continued to work on mechanical tweaks in bullpen sessions with pitching coach Dave Eiland, and his results improved. Chamberlain wrapped up the Grapefruit League with a 3.60 ERA, allowing eight runs and 20 hits in 20 innings, striking out a batter per inning.

"It got a lot better," Chamberlain said. "I continued to learn. The first two were terrible, and we got going as it went. This was a good one to build off as far as velocity and attacking the zone with fastballs. I threw good sliders and a couple of good changeups. All in all, it was the best."

Rumors of Chamberlain's dwindling velocity were dismissed by Tuesday's performance, when radar guns showed readings between 90 and 96 mph. Girardi said that clocking Chamberlain was not his main concern this spring.

"It wasn't a huge concern to me, because he had been at 95 [mph]," Girardi said. "I know some people were concerned about it, but I figured when he got a little bit stronger and we got a little closer, we'd see his velocity."

Girardi said that Chamberlain's dominating history as a setup man may have set the bar too high.

"People expect him to pitch that way every inning as a starter," Girardi said. "It's not going to happen. Usually when starters go to the bullpen, their velocity goes up a little bit.

"When they go back to the starting, it's down a little bit on a consistent basis. Joba has so many expectations on him, I'm not sure he can live up to the expectations."

As a starter last season, Chamberlain showcased some of that promise, faring 3-1 with a 2.76 ERA in 12 starts for New York, including seven innings of scoreless three-hit ball on July 25 at Boston's Fenway Park.

"I felt good about him as a starter," Girardi said. "His ERA was excellent, and he had a chance to win a number of games for us. In [the American League East], you need five quality starters. We feel Joba gives us another solid starter who can pull his weight."

Posada said that he has seen a live arm that will help the Yankees, not only this year but in the future. There will be bumps; in the third inning, Chamberlain had two outs and a 1-2 count on Reds first baseman Joey Votto.

Chamberlain wanted to throw a changeup, and Posada shrugged. Let him see what happens, Posada thought, moments before Votto sharply singled to right field. Were it the regular season, Posada said he would not have called for what he doesn't consider to be Chamberlain's best pitch.

"He's going to get stronger and he's going to learn, and he's going to get a lot more aggressive," Posada said. "That's what we want. We want him to learn. We've got some great starters in front of him that are going to help him get where he wants to go."

While the Yankees plan to closely monitor Chamberlain's innings, Girardi said they would manage the timing of his starts, not pull him out of games early because of pitch counts. But it is possible Chamberlain could be skipped in the rotation at opportune times.

"As a competitor, that's going to [stink]," Chamberlain said. "I'm going to be honest with you, you want to be in the game as long as you can.

"Your teammates are battling, but I also understand they're going to watch out. I'm going to look them in the eye and say I don't like it, but there's not much I can do about it."

Girardi understands that the debate is not going away anytime soon, continuing to spark fodder on sports-talk radio and the blogosphere. But the decision is final -- Chamberlain is the Yankees' fifth starter and someone else will have to handle getting the ball to closer Mariano Rivera.

"You're going to have to win a lot of games to win this division, and we've always considered him a starter," Girardi said. "So that's what we're going to do with him."

Bombers bits: Rivera threw 21 pitches in a Minor League game in Tampa, Fla., recording five outs. He allowed a home run, but he struck out two. ... Hideki Matsui was scheduled for a day off on Tuesday and came in with a stiff neck. ... Xavier Nady suffered a bruised left elbow when he was hit by a Micah Owings pitch in the sixth inning on Tuesday. He left the game, but he was said to be fine.