In 1996 Mariano Rivera relieved in 26 of the 43 games John Wetteland saved for the Yankees. Rivera, however, was not simply an eighth-inning setup reliever for Wetteland. Only in 6 of those 26 games did Rivera pitch only one inning.

Twice Rivera pitched three innings. He pitched two innings 13 times. He pitched between two and three innings three times.

That season was clearly the beginning of the idea of shortening a game to six innings. The Yankees had thoughts of revisiting or recreating 1996 with Rivera playing Wetteland and Joba Chamberlain in the Rivera role. It will not happen.

Rivera is ready for his role. He has played it for 14 years in the most remarkable relief run in history.

Chamberlain, though, is missing. A pitcher whom the Yankees have shuffled between starting and relieving in his three major league seasons, Chamberlain has stumbled backward in his career and in the Yankees’ plans for him.

He has fallen from regular starter in 2009 to promising reliever last year to reliever in an undefined role this year. Ideally he would have been the setup man for Rivera this year and next, then replace Rivera as the closer in 2013.

But the 24-year-old right-hander had lost so much of his early luster that the sons of Steinbrenner told general manager Brian Cashman that they preferred spending more money and having a larger payroll than the alternative.

“I was prepared to have him compete with someone else who is a real good relief pitcher, David Robertson,” Cashman said in a telephone interview Wednesday. “That isn’t something I was afraid of. The player we added is pretty spectacular in his own right.”

Hal and Hank Steinbrenner didn’t have a lot of confidence in Chamberlain or Robertson, a 25-year-old right-hander. Last season was Robertson’s first full season in the majors, and the Steinbrenners weren’t about to gamble with him as the setup man.

That left no choice as far as the Steinbrenners were concerned. Despite Cashman’s reluctance to lose the Yankees’ No. 1 draft choice next June as compensation, the Yankees signed a free agent, Rafael Soriano, for three years and $35 million.

American League leader in saves last season with 45, Soriano will be Rivera’s setup man for two years, then replace him in 2013. Where does that leave Chamberlain? In a reduced role out of the bullpen. If a starter doesn’t do his job, someone has to pitch the sixth and seventh innings.

The Yankees feel Chamberlain hasn’t been the same since he hurt his shoulder in August 2008. “He does not throw the way he used to,” Cashman said. “He used to throw between 93 and 99. Now he throws between 89 and 93. He’s not the guy he was. But in the bullpen he can still hump it up. We believe he’s a reliever now. We wanted to see what he could do as a starter.”

But others feel the Yankees created a problem by moving him back and forth between starting and relieving.

A starter in the minors, Chamberlain earned a quick call up in 2007, his first year as a professional pitcher, and relieved in all 19 games he pitched. There was nothing unusual about that. Many pitchers break into the majors as relievers, then move into the starting rotation.

“The history of our game has been starters breaking in to the major leagues as relievers,” Cashman said. “The bottom line is if you’re good enough to do something you will. If not you’ll settle into something else. Joba is a very capable relief candidate. He had a good year last year. He had some bad outings that blew up his e.r.a.”

In 2008 Chamberlain performed split duty, starting 12 games and relieving in 30. The next season Chamberlain found himself in the starting rotation the entire season. He responded with a 9-6 record and a 4.75 earned run average. As a starter, the Yankees had him on a strict schedule, making sure he didn’t pitch too many innings (he pitched 156 1/3 in 31 starts, averaging 5 per start).

By last season, however, the Yankees decided Chamberlain should work out of the bullpen, and he relieved in 73 games, tied for fifth most in the league, though he pitched only 71 2/3 innings and worked two innings only once.

In 1996, Rivera relieved in 61 games and pitched 107 2/3 innings, working two or more innings 35 times. But he had been a starter in the minors, making 51 successive starts in four seasons before his 1995 promotion to the Yankees, for whom he started 10 games and relieved 9 times.

There’s no way of knowing for certain, but Chamberlain has probably suffered from the way the Yankees have moved him back and forth.

“No question back and forth can have an effect; with power type guys going back and forth can be an issue,” said a pitching expert I spoke with. “His velocity dropped when he was a starter and spiked back up when he went back to the bullpen.”

Now that he’s in the bullpen, Chamberlain will most likely stay there, as long as he remains with the Yankees. If they trade him, which is entirely possible, another team could try him as a starter.

One problem with Chamberlain the starter was he was too emotional to be successful.

“You can’t maintain that emotion for an entire game,” said the expert, who spoke on the condition of anonymity because he was speaking about another team’s player. “As a starter, you can’t go with that intensity for 100 pitches. You’ll burn yourself out emotionally.”

In addition, he said, “From what you can observe of Joba he’s not the most cerebral guy.”

Phil Hughes, the Yankees’ other young pitcher who has started and relieved, is better equipped emotionally to start, the expert said, and indeed Hughes had an 18-8 record last season after having pitched primarily in relief the previous year.

Compared with Chamberlain’s fist-pumping, vein-popping, screaming histrionics, Hughes pitches in a trance.

The expert did not fault the Yankees for starting Chamberlain or Hughes.

“Buying starting pitching in the free agent market is expensive,” he said. “The way economics are you have to find out if guys can start. You have to explore that. With pitching today you almost have to try everybody as a starter. They went through the exercise of finding out and they found out. They found that Hughes could do it.”

But now they can’t be sure what Chamberlain can do.