Specifically, an offense that hasn't offered much run support in Peavy's starts continued to struggle with runners in scoring position as the Sox loaded the bases with no outs twice but managed only two runs.
The Sox's deficit in the American League Central swelled to two games behind the Tigers with six games remaining.
And with the Sox losing for the eighth time in nine games, regaining momentum seems as daunting as catching the Tigers.
"We had it right there in front of us tonight, and through a lot of the last 1 1/2 weeks," Peavy said. "I just couldn't be good enough."
Peavy, who is not likely to have his $22 million option picked up by the Sox and instead likely to receive a $4 million buyout, threw 7 1/3 innings of four-hit ball and preserved most of a taxed bullpen.
Pitching on five days' rest, Peavy showed his strength by striking out four of the first six batters he retired.
But he was tagged for a run in the fourth on Carlos Pena's sacrifice fly, and Luke Scott opened the fifth with a home run.
The Sox unraveled offensively in the eighth.
After singling to start the inning, A.J. Pierzynski was pulled in favor of pinch runner Jordan Danks.
Dewayne Wise was summoned to pinch-hit for Dayan Viciedo, but Wise failed on two sacrifice bunt attempts before striking out. Alexei Ramirez hit a deep fly to center for the second out, and Danks failed to re-tag second base while retreating to first and was called out for the third out.
Longoria cranked his 14th homer to give the Rays their eighth consecutive victory and their 13th triumph of the season in their final at-bat.
"We weren't doing the small things to win games," manager Robin Ventura said. "Over the course of time, it builds up."
The Sox, who were batting .179 with runners in scoring position in their previous 20 games, continued to minimize opportunities. They were 1-for-8 in those situations.
That was clear when they loaded the bases with no outs in the fourth and fifth innings.
Their first run came in the fourth after Pierzynski swung wildly and struck out for the first out.
Dayan Viciedo was hit by a pitch near the midsection to force in a run, but the Sox failed to add any more.
In the fifth, the top of the order loaded the bases. But the only run scored when Paul Konerko grounded into a double play.
Usually dependable Alex Rios struck out to strand Kevin Youkilis at third.