MLB

JOBA HURT

ARLINGTON, Texas – Joba Chamberlain spitting out a three-run lead in the Texas heat wasn’t the worst thing that happened to the Yankees last night.

Nor was it Chamberlain getting spanked by the muscular Rangers, who started the sweltering evening as the best hitting team in the majors.

And as much as it stung, Marlon Byrd’s game-winning grand slam in the ninth inning didn’t cut the deepest, either.

No, Chamberlain being walked off the mound by trainer Steve Donohue in the fifth inning chilled the spine of every Yankee fan watching the Rangers’ 9-5 victory in front of 33,813 at Rangers Ballpark.

The Yankees announced Chamberlain left because of a stiff right shoulder and gave no reason for the problem. They said after the game that he would be sent home for tests.

The injury could have stemmed from Chamberlain ducking and taking a tumble while getting out of the way of a Pudge Rodriguez throw to second in the fifth. The catcher thought he had a double play chance on Ian Kinsler’s dribbler in front of the plate, and threw to second, narrowly missing Chamberlain’s head. The ball, however, was ruled foul.

It’s also possible Chamberlain was dehydrated. After all, the game-time temperature was 101 degrees.

Whatever the reason, the Yankees missed the chance to gain ground in two races because AL East leader Tampa Bay lost to the Indians and the Red Sox, who started the night on top of the wild card race, were defeated by the Royals. The Yankees are 5½ games behind the Rays and 2½ back of the Red Sox.

Thanks to five straight quality starts and his overpowering arsenal, Chamberlain was on the cusp of being called the Yankees’ ace. Then he got spanked and left a game with an injury.

Those who said Chamberlain was more valuable to the Yankees as a reliever than a starter based their argument on his violent delivery having the potential to lead to an injury.

In 4 innings, Chamberlain gave up five runs and eight hits, two of which were homers.

Byrd sent the Yankees to defeat with a two-out, grand slam in the ninth off Damaso Marte, who loaded the bases by issuing three walks. Byrd sent the first pitch he saw over the center-field fence.

Xavier Nady’s fourth homer as a Yankee went to right field in the eighth and tied the score, 5-5. It also allowed Chamberlain avoid a loss.

Jason Giambi, newly shorn of his mustache, and Robinson Cano also homered for the Yankees.

Marte took over for Brian Bruney with the bases empty and one out in the eighth and gave up a triple to left-handed hitting David Murphy. With the infield in, Marte fanned another lefty, Chris Davis, for the second out. Switch-hitter Jarrod Saltalamacchia, a weaker hitter from the right (.167) side than left (.280), flied to center.

In the fateful fifth, Chamberlain gave up a leadoff single to Ramon Vazquez, the No. 9 hitter, and walked Kinsler after the double-play was waved off. Gerald Laird’s fly to left was followed by Michael Young’s opposite-field, three-run homer that put the hosts ahead, 5-4. Chamberlain fanned Hamilton and gave up a single to Byrd before his shoulder acted up. He was lifted after Donohue and manager Joe Girardi visited the mound.

Rangers starter Vicente Padilla committed two balks in the fifth that allowed Johnny Damon to go from first to second and third to home and resulted in Rangers manager Ron Washington getting ejected.

Damon led off with a single, was balked to second, went to third on Bobby Abreu’s grounder to the right side and scored when first base umpire Alfonso Marquez called Padilla for a balk. Washington raced out of the first-base dugout, put on an animated argument and was tossed.

george.king@nypost.com