Lefty starter Felix Doubront, filling in for the rehabbing Clay Buchholz, made two costly throwing errors, the latter of which led to Molina's game-breaking grand slam in the top of the fifth.
Though they swung the bats fairly well, the Red Sox lacked the big hit when they needed it and lost for the seventh time in the last nine games.
There is no overstating the fact that the entire game changed in the top of the fifth inning. With two on and one out and the Red Sox in possession of a 3-2 lead, Josh Hamilton hit a rocket back to the box. Doubront snared it out of the air in self defense, and he thought he had Michael Young doubled off second base.
Doubront hesitated for a second while Marco Scutaro covered the bag, but the lefty didn't have his legs under him when he made a wild throw into center field, pushing the runners to second and third.
"Great stab," said Red Sox manager Terry Francona. "He just didn't move his feet. That's a big play. We talk all the time about giving them extra opportunities. That probably played itself out to the worst it could. Got a chance to walk off the field, which would have been a great play. Instead, have to keep letting them hit it and it unraveled."
Yes, the unraveling was severe. Doubront, who sat through a one-hour rain delay in the top of the fourth, came out after that ill-fated error. Fernando Cabrera, called up from Triple-A before the game to give the bullpen an extra arm, walked Nelson Cruz and David Murphy, the latter of which forced in the tying run.
Then, Molina's night started building toward epic status as he clubbed a grand slam to center. Molina became the first opposing player to hit for the cycle at Fenway since Andre Thornton did it for the Indians on April 22, 1978.
With the single (second inning), double (fourth inning) and homer out of the way, the heavy-legged catcher hit just his sixth career triple to lead off the eighth, completing the cycle.
"What can I tell you? I tell people that they think they know everything about the game," said Red Sox slugger David Ortiz. "Nothing is predictable in the game. Things just happen. I'd put my head in a tree trimmer betting that he won't hit a triple. That's crazy, man."
Molina's triple was just off the glove of Red Sox center fielder Eric Patterson, before rolling into the triangle-area in right-center -- the deepest portion of Fenway Park.
"You have it in your head, but you're not thinking about hitting a triple," said Molina. "Just hit the ball hard. But when he dropped the ball, that's when you're thinking, 'Got to go, Got to go.'"
It took a toll, as Molina had to leave the game with discomfort in his right quad.
"It seems to me like he stopped at second to think about it," Ortiz said. "That's crazy. Unbelievable. Now, I'm going to have to break that down for my son when I get home, because he's going to ask me a million questions about it."
The Red Sox now trail the Yankees by 6 1/2 games in the American League East, their largest deficit since May 29. Boston is 3 1/2 back of the Rays in the AL Wild Card standings.
As for Friday's plight, the five-run fifth was a pure downer for the Red Sox, who had just taken the lead on Adrian Beltre's solo blast in the bottom of the fourth.
"It just didn't happen," said Doubront. "Sometimes it's good, sometimes no. It's all right."
Doubront could be optioned back to Triple-A Pawtucket as early as Saturday, with Buchholz expected to return to the rotation on Wednesday at Oakland.
The night got off to an ominous start for the Red Sox, when Rangers leadoff man Elvis Andrus led off the game with a grounder that Doubront picked up and threw down the right-field line. A two-run inning ensued.
"The first throw, that's one that's probably rushed, but there's a reason to rush it because of the runner," Francona said.
The Red Sox bounced back in the bottom of the second. Kevin Youkilis led off with a double off the Monster, and J.D. Drew belted an RBI single to right. Though Boston went on to load the bases with nobody out, they would score just once more in the inning -- on a wild pitch.
In the end, however, there was just no stopping Molina.
"The last two nights, every mistake we make, he hits," Francona said. "He's knocking the ball all over the ballpark. He's kind of hiding out down there a little lower [in the order] and whacking the ball all over the place. He killed us."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.