ST. PETERSBURG -- Unlike the last time the Red Sox lost a Sunday night game at Tropicana Field, their season didn't end on this occasion. This wasn't Game 7 of the American League Championship Series, like it was back in 2008. But it was another stinging defeat in a game the Red Sox badly wanted to win.

A weekend of hope slipped away from the Red Sox, as they suffered a 5-3 loss to the Rays in the rubber match of a critical three-game series against one of the teams they are trying to catch in the standings.

"It's tough," said Red Sox third baseman Adrian Beltre. "We came hoping to win the series and it didn't happen. It's in the past and we'll do whatever we can do from now on to gain some ground."

After winning the opener on Friday night, Boston lost the last two to fall 6 1/2 games behind both the Yankees and Rays in the American League East.

Making it tougher was that the Red Sox were in front in the mid to late innings of both games.

"I mean, what are we five or six behind? We've got a lot more games than that [left]," said righty John Lackey, who took the loss. "I'm not a math whiz, but I think it's doable."

From the managers' office to the various lockers in the clubhouse, there was no lack of resolve from the Red Sox.

"Well, there's still 30-something games left," said Red Sox catcher Victor Martinez. "It's not over yet, it's not over yet. We come back Tuesday in Baltimore, keep playing hard, man. That's the only thing we can control, and [we'll] see what happens."

That didn't make the finale at the Trop any less painful, however.

Lackey was staked to a 3-1 lead heading into the bottom of the sixth, but he gave up three runs in that frame and the Rays never trailed again. It was a disappointing start for Lackey, who gave up nine hits and five runs over 6 1/3 innings, walking three and striking out seven.

"I'm still trying to figure it out, for sure," said Lackey. "I mean, I felt way too good to give up five runs."

The big righty, who signed a five year, $82.5 million contract back in December, is 12-8 with a 4.60 ERA.

"I mean, it's frustrating," Lackey said. "There's been a lot of them, but turn the page and keep grinding."

The Red Sox, who have Monday off, have 31 games left in their season, starting with the opener of a three-game series in Baltimore on Tuesday.

"Again, what happened tonight is a disappointment -- we lost," said Red Sox manager Terry Francona. "Now we'll regroup, take our day off and see if we can win the next game. That's the best way to do it. That's why we talk about it."

Rays starter James Shields and Lackey were both in top form early. But Lackey cracked first, leaving a 91-mph fastball right down the middle, and Carlos Pena crushed it for a solo homer to right-center with two outs in the bottom of the fourth.

But the Red Sox came right back against Shields in the top of the fifth. Mike Lowell got things started with a double to right-center. Daniel Nava moved Lowell to third on a groundout to second. After a walk to Darnell McDonald, Yamaico Navarro came through with an RBI single to left. Navarro was making just his second Major League start.

"We're trying to win," Francona said. "We're just trying to find ways to win. We played him because we thought he could help us win."

Marco Scutaro then stung a single off the glove of third baseman Evan Longoria and into left to score McDonald and Boston had a 2-1 lead.

It was Beltre who ignited Boston in the sixth, hitting one down the line in left and hobbling on his bad left hamstring with a double. With two outs, Nava came up with a clutch RBI single to right with Beltre again hustling around despite obvious discomfort and scoring from second. That gave Lackey a two-run edge.

However, there was a jarring momentum swing in the bottom of the sixth. Ben Zobrist hit a single up the middle and Carl Crawford smashed a game-tying, two-run blast to right.

"I mean, the home run, I went with a slide step on that, and didn't quite get the ball down and away like I needed to get it," said Lackey.

Making matters worse, Lackey couldn't stop the bleeding after the Crawford homer. Longoria followed with a double to right-center. The Red Sox then ordered for an intentional walk of Pena. Lackey walked Matt Joyce to load them up with just one out. And Dan Johnson, the walk-off hero from Saturday, stung an RBI single up the middle that could have been two runs if not for a terrific block of home plate by Martinez.

Martinez got drilled in the left leg by Pena, but held on and stayed in the game after a meeting with Francona and a member of the training staff.

"He just got me right in my foot, but everything is fine," said Martinez.

For Lackey, it was less than fine.

"Honestly, I gave up like three hard-hit balls all night, and they pretty much maximized the damage they could get," said the righty.

The Rays added some insurance in the seventh, as Longoria belted an RBI single up the middle against Hideki Okajima, who was making his first appearance since being activated on Saturday.

There were no late-inning dramatics for the Red Sox, who will see the Rays again Sept. 6-8 at Fenway.

"We don't need to think about them right now," said Francona. "That series is whenever it is. We better go concentrate on Baltimore."

And on the way out of Tropicana Field for a late-night flight that would take them to the Charm City, the Red Sox made sure to emphasize time and again that it is too early to count them out.

"Not yet," said Ortiz. "Not yet."