MLB

Suddenly, Joba and Mo are no guarantees

The Yankees never have worried about Joba Chamberlain’s talent. It is overt, the blunt force of 95-plus-mph heat.

The concern always has been about health, hence the Joba Rules. But something else also has nagged at the organization. The Yankees privately have expressed concern that Chamberlain has a tendency to get comfortable, especially at times of success. And at those moments his focus drifts and his performance drops.

It happened to him, Yankees officials believe, as a starter when he would go from his best spurts almost seamlessly into periods when his concentration and velocity would wane. And it sure appears as if it is occurring now, as well.

“You can’t get comfortable or it will bite you,” Chamberlain said at the end of a long, cold and ultimately frustrating night for the Yankees.

The Red Sox rallied for a 7-6 triumph because for the second time in three days neither Chamberlain nor Rivera was effective. Maybe that is just a blip for two guys who were going so well.

But, again, the problem with Joba tends to be that when he is going well he begins to, perhaps, relax or take too much for granted. He had earned a win Friday night against the Twins, striking out the side in the eighth. At that moment, he had struck out seven of his last 10 batters and had not permitted a run in eight straight outings. He had a 2.16 ERA on the season, a .197 batting average against and looked like an eighth-inning weapon now and Riv era’s heir apparent later.

Since then, boom, he has looked like the second coming of Jay Witasick, seven runs against five outs over his last two appearances. He has morphed from impregnable to as big an issue as the disappointing-so-far David Robertson and Chan Ho Park. The Yankees had believed that their pen was going to be a strength, but at this moment they are without a trustworthy cog, with the eighth inning, in particular, becoming a horror show. This is the eighth inning from the past three games:

On Sunday, the Yankees led 3-1, but Joba and Rivera combined to permit five runs and the Twins won 6-3.

On Monday, the Yankees led 7-6, but with Chamberlain unavailable, Park gave up three runs. The problem was somewhat blunted because the Yankees rallied in the bottom of the ninth to score four runs and beat the Red Sox 11-9.

Last night, however, there was no camouflage for the growing pen trouble. Chamberlain came in with a 5-1 lead, the Yankees six outs away from a two-game sweep that was going to drop Boston two games under .500, further dent the Red Sox’s psyche and move the Yankees into a series against the Rays on a high.

“This is my loss, nobody else’s,” Chamberlain said.

Alex Rodriguez made a throwing error to open the eighth and Chamberlain could not pick up his third baseman. He gave up three straight hits, got Victor Martinez to ground out on a 3-0 pitch and then had David Ortiz launch a game-tying single off the wall in right.

“I have to be better with my fastball command,” said Chamberlain, who left to boos. “I have to make better pitches.”

Still, the score was tied when Rivera entered in the ninth. But a one-out, one-on error by Marcus Thames opened the way for a two-out, two-run double from Jeremy Hermida that gave the Red Sox the lead.

The previous day A-Rod and Thames had been the heroes, their two-run, ninth-inning homers covering up for bullpen malfeasance in a come-from-behind Yankees win. Last night their errors were not something Joba or Rivera could overcome.

Maybe it is just a blip. It has proven folly in the past when a brief rough spot has motivated “Is this the end?” questions about Rivera. He gets the benefit of the doubt.

Chamberlain does not. He has this stigma of getting comfortable that he must disprove. The Yankees are reeling in the eighth inning right now. Does Joba have the maturity to stay laser-guided to the mission all year and be the no-questions-asked, set-up answer?

joel.sherman@nypost.com