The Red Sox’ David Ortiz reacts with disgust as he strikes out on a pitch by Blue Jays’ starter A.J. Burnett in the first inning last night at the Rogers Centre in Toronto.

AP / Adrian Wyld

TORONTO — To succeed in the postseason, the Red Sox firmly believe they need an effective Eric Gagne in the late innings.

But while they showed patience in the veteran reliever last night, their short-term prospects took a serious blow.

Gagne turned a one-run lead into a two-run deficit in the eighth inning, resulting in a crushing 4-3 loss to the Toronto Blue Jays, enabling the surging New York Yankees to climb within 2½ games of the Sox in the American League East.

Gagne, obtained in a July 31 deadline deal, retired the first two hitters in the eighth before issuing a walk to Frank Thomas. After a single to left by Aaron Hill, Gagne got ahead of Matt Stairs 0-and-2 before walking him to load the bases.

He then walked Gregg Zaun to force in the tying run, then surrendered a two-run double that eluded J.D. Drew in right.

A two-out solo homer from Julio Lugo in the top of the ninth wasn’t enough for the Sox to complete the comeback.

“I don’t know how to put it into words,” said a downcast Gagne, his voice barely above a whisper. “I felt good physically. But I walked Thomas and after that, I couldn’t throw a strike. You can’t get people out if you don’t throw strikes.

“It’s a little bit of everything. I don’t know. I’ve got to go out and stop thinking. It’s frustrating. I don’t know what to tell you.”

Manager Terry Francona got closer Jonathan Papelbon up after the single by Hill, but left Gagne on the mound as the lead was frittered away.

“It seemed like (after walking Thomas), it looked like he wanted to throw the ball through the backstop,” said Francona. “He got a little revved up and it didn’t end very well.”

Francona said he resisted going to Papelbon because “that’s (Gagne’s) inning to get out of. There are a lot of long-term reasons to keep him out there and have success. When it doesn’t work, it hurts. … We just have to get it right. We want him to give us strong innings, so we’ve got to just get it right.”

“We need him these last two weeks to get ourselves where we want to be,” echoed catcher Jason Varitek.

Last night marked the third time in the last five games that the Sox have let a game get away from them in the eighth inning.

They led the Yankees, 7-2, Friday night before Hideki Okajima and Jonathan Papelbon imploded and the Sox lost, 8-7. Sunday night, Curt Schilling yielded a three-run homer to snap a 1-1 tie in the eighth.

“Those happen (to all teams) at some point,” said Varitek. “You don’t want to see it happen now. Now, we have to gut it up and get it done.”

The loss spoiled a terrific start by Jon Lester, who pitched 6 2/3 innings and allowed a single run on three hits.

Lester needed 28 pitches to get through the first inning, when he issued two walks and allowed two hits. But after the first, he retired 16 of the next 18 hitters he faced and the Jays didn’t get a runner into scoring position.

“He got himself into a bind,” said Francona, “and then was able to get out of it. As he commands, he can be a really good pitcher.”

The Sox couldn’t do much against A.J. Burnett, but until Gagne’s struggles, they did enough.

In the fourth, Varitek, who snapped an 0-for-18 slump with a second-inning single, stroked a double into the left-field corner, scoring Mike Lowell who had reached on an infield single and advanced to second on a walk to J.D. Drew.

Back-to-back two-out hits in the fifth — a single by Dustin Pedroia and a double to center by David Ortiz — gave the Sox their first lead of the night.

But after the double by Ortiz, the Sox didn’t collect another hit until Lugo’s line-drive homer to left with two outs in the ninth. Burnett fanned two Sox hitters in the sixth and seventh each and finished with 11 strikeouts.

Scott Downs came in for the final out and caught Jacoby Ellsbury looking at a called third strike.

smcadam@projo.com