ST. PETERSBURG, Fla. — When the Yankees have all their biceps to flex, giving up three runs in six innings usually doesn’t get a pitcher beat.
Yet, until the Yankees begin to heal, the margin of error for their hurlers is a lot slimmer than it is with Curtis Granderson, Mark Teixeira, Derek Jeter and Alex Rodriguez in the lineup.
Add a brutal error by right fielder Brennan Boesch to a lineup that was without Kevin Youkilis and dominated by right-hander Alex Cobb and you understand why Andy Pettitte absorbed a 3-0 loss Wednesday night to the Rays in front of 19,177 at Tropicana Field.
Pettitte allowed three runs, seven hits, hit a batter and fanned 10 in six innings on the way to his first loss of the season in four decisions.
Cobb went 8 ¹/₃ innings, allowed three hits and improved to 3-1.
Fernando Rodney retired Robinson Cano and Travis Hafner with two runners on in the ninth for the save.
Boesch’s error on a routine ground single to right fueled a two-run fifth inning that included Pettitte hitting Jose Molina and Ben Zobrist delivering a two-out, two-run double. Sean Rodriguez upped the lead to 3-0 in the sixth with a leadoff homer to left-center.
Cobb, a 25-year-old right-hander who made his first Opening Day roster this year, continued to handcuff Yankees hitters. The current roster were a combined 6-for-35 (.171) against him.
Pettitte’s outing was the shortest of the four starts this year but still good enough to win had the Yankees collected more than two hits.
“If you look at the one thing he has done really well, he has given us distance,’’ manager Joe Girardi said of Pettitte before the game.
After taking two of three games in the previous three-game series, the Yankees dropped two of three to the Rays.
Rodriguez’s first homer of the season led off the sixth against Pettitte and upped the host’s advantage to 3-0. Rodriguez hit Pettitte’s initial pitch off the top of the center-field fence and the ball glanced over the wall.
Yunel Escobar followed with a sharp single to left to bring the right-handed hitting Shelley Duncan to the plate. With right-hander Shawn Kelley throwing in the bullpen, Girardi went to the mound but it wasn’t to hook Pettitte.
Instead, the manager talked to the pitcher and left Pettitte in to face Duncan, the ex-Yankee, who walked in the second and fanned in the fourth. Pettitte rewarded Girardi’s confidence by striking out Duncan.
Eduardo Nunez went deep into the hole to rob Jose Molina for the second out and Pettitte kept the deficit to three runs by getting Kelly Johnson on a grounder to the right side.
george.king@nypost.com