BOSTON — “Everyone’s entitled to a slump,” Red Sox general manager Theo Epstein was saying prior to last night’s game when it was pointed out that David Ortiz has not been slugging the ball this season quite the way he had in the past.

Going into this three-game set with Baltimore, Boston’s designated hitter had amassed only 16 home runs. He had gone 38 at-bats since his last one, which came on July 16 against Kansas City, hitting just three homers over the course of the last 29 games, a total of 116 at-bats.

Nevertheless, insisted Epstein, “David is having a great year,” pointing out also that Ortiz has been playing with a torn meniscus in his right knee. “He may not be putting up numbers (he has in the past), but he has battled through some leg problems and seems to be better lately. He’s not a complainer. I wouldn’t bet against him. He’s doing everything he can.”

And Ortiz certainly demonstrated that last night, accounting for three of Boston’s four hits and delivering his 17th and 18th homers in a 5-3 setback to the Orioles before a Fenway crowd of 36,866.

It marked the 28th multi-home run game of Ortiz’s career — his 26th with Boston — and his second of the season. (He also hit two homers at Texas on April 8.)

“That was exciting when he starts leaving the ball park,” said Sox manager Terry Francona. “The way he battled in his first at-bat [a nine-pitch at-bat in which Ortiz ended up with a bloop single to left]. He saw every pitch [Baltimore starter Erik] Bedard had, and Bedard is as tough on lefties as anyone. You could see David feeling better as the night went on.”

Ortiz, who hit a club-record 54 home runs last season, collected No. 17 in the third inning, crushing a curve ball from Bedard into the right-field seats for a two-run homer. It was his first of the year against a left-handed pitcher.

Ortiz’s second home run of the game, against Orioles reliever Rob Bell, came in the eighth, when he parked a solo blast into the Sox bullpen to make it 5-3.

Unfortunately for the Sox, Ortiz’s offensive production, which boosted his average from .316 to .321, wasn’t enough to get them past Baltimore, which has gone 12-5 since the All-Star break and is coming off a 5-1 homestand that included a three-game sweep of Tampa Bay and 2 of-3 against the Yankees.

Josh Beckett, who was tagged with a 1-0 loss to Cleveland last Thursday despite producing what catcher Jason Varitek called “one of the best outings Josh has ever had,” took the loss again last night, after giving up five earned runs on nine hits over eight innings.

“It was one of those deals where it seemed like every time they hit the ball, it fell in,” said Beckett. “Just one of those nights where you suck it up, take your whipping and go home. You go home and think about how bad you were.”

Brian Roberts wasted no time putting Baltimore on the board, driving the first pitch of the game off the right-field wall and into the stands.

Roberts then doubled off the left-field wall to drive in Jay Payton for the first of three more Orioles runs in the third.

A shot up the middle by former Red Sox Kevin Millar eluded a diving Julio Lugo, which allowed Roberts and Corey Patterson, who had reached on a walk, both to score.

Boston got two back in the bottom half of the inning on Ortiz’s two-run blast.

But the Sox then wasted a prime opportunity to push a few more runs across in the fourth after loading the bases with one out when Bedard issued a walk to Kevin Youkilis, hit Jason Varitek with a pitch and then walked Coco Crisp. They were all left stranded, as Wily Mo Pena struck out swinging and Julio Lugo was called out on strikes.

Beckett then gave up back-to-back singles to Miguel Tejada and Aubrey Huff, followed by an RBI double to Ramon Hernandez in the sixth to make it 5-2.

After Ortiz got Boston within two in the eighth, the Sox brought the winning run to the plate in the ninth. But with Crisp on third and Lugo on first with two outs, Orioles lefty closer Jamie Walker forced Dustin Pedroia to ground out back to the mound, picking up his fourth save and helping Baltimore secure its 8th win in 10 games.

cthorn@projo.com