“He kept them at bay for a while,” Red Sox manager Bobby Valentine said.

The problem with a sinkerball pitcher is what happens when the ball is misplaced in the strike zone, even just a little. Then one of the best pitches in baseball becomes one of the easiest to hit.

Cook had a lead and couldn’t hold it. Long home runs by Edwin Encarnacion and J.P. Arencibia helped give the Blue Jays a 7-3 victory before an announced crowd of 38,170.

The Sox won five of their first seven games coming out of the All-Star break, but have since lost two games to the last-place Jays. At 48-47, the Sox’ seasonlong tango with .500 continues.

Now struggling Jon Lester takes the mound Sunday as the Sox try to take one game in the series. A crucial road trip to Texas and New York follows.

Come this time next week, the puzzle of whether the Sox are actual contenders should be a little easier to solve.

“We feel like we have a great team. We just need to be more consistent,” Dustin Pedroia said, repeating a theme that hasn’t taken hold. “Be consistent offensively, pitching, running the bases, playing good D. We do everything better, we’re going to run off a lot more than five or six in a row. We need to do that.”

Cook (2-3) took a 3-1 lead into the sixth inning then got two quick outs. He had retired 11 straight at that point.

Colby Rasmus walked on five pitches, Cook suddenly losing command. The next hitter, Encarnacion, fell behind 1-and-2 then sent a sinker over everything in left field for his 26th home run.

Cook kept the pitch down, but it was right over the plate. Encarnacion adjusted his swing, taking more of an uppercut to get the ball in the air. As he dove for the pitch, it came right to him.

“Walking [Rasmus] before the home run, that’s the real killer,” Cook said.

It was the third home run Encarnacion has hit against the Sox this season. It also gave him 11 RBIs in 34 at-bats against the Sox.

Arencibia started the seventh with a long home run to left field. Cook left the game with one out and Yunel Escobar on first after an error by Will Middlebrooks at third base.

“They only hit two balls hard against him. They both went over the fence. It was a pretty good outing,” Valentine said.

Said Cook: “Those two bad pitches, they made me pay for them tonight.”

The game got away from the Sox once Cook left the mound. Franklin Morales struck out Travis Snider but walked pinch hitter Rajai Davis. Matt Albers was next out of the bullpen and got ahead of Brett Lawrie 1 and 2. But he could not put him away.

After a double steal by Escobar and Davis, Lawrie grounded a two-run single into left field. Carl Crawford’s throw to the plate, while accurate, was too late to get Davis.

The Jays added another run off Junichi Tazawa in the ninth.

The Sox offense consisted of three swings in the second inning against Carlos Villaneuva (5-0).

Adrian Gonzalez started the inning with a soft single to right field. Cody Ross followed with a double to left.

Jarrod Saltalamacchia fell behind 0 and 2 then lined a good pitch, a low slider on the inside corner, into the Toronto bullpen. It was the 18th home run of the season for Saltalamacchia, the most among major league catchers.

Saltalamacchia had come into the game hitless in his previous 14 at-bats and 4 for 41 this month with 22 strikeouts.

“Perfect. Just what the doctor ordered for Salty,” Valentine said. “And it came with two strikes, and he laid off a good pitch down in the zone.”

But that was it for the Sox. Villaneuva and three relievers allowed just two other hits. Villaneuva went 6⅓ innings, allowing four hits. He is 4-0 as a starter since joining the Toronto rotation in late June.

With Jacoby Ellsbury held out to rest after playing eight games and David Ortiz on the disabled list, the Sox were listless.

The offense has been sporadic of late. The Sox scored three runs Thursday, all on a walkoff home run by Ross. They then scored one run Friday and three Saturday.

“We’ve got to add on, that’s for sure. We’ve got to put stuff together,” Valentine said.

On a night when the Red Sox honored his mentor, former team captain Jason Varitek, Saltalamacchia tried to sound a positive note.

“We just got to continue to grind,” he said. “We’ve got to go out there, get some pitches to hit, do a better job at that.”

Peter Abraham can be reached at pabraham@globe.com. Follow him on Twitter @PeteAbe.