"Left a curveball middle-up. [Omar] Vizquel has been around a long time, didn't try to do too much with it and got a base knock," Lester said about Vizquel's game-winning RBI single.
Although the Red Sox's postseason aspirations were dashed following last month's blockbuster trade with the Dodgers, Boston was officially eliminated from the playoff chase with the loss.
Lester retired the first two batters he faced in the seventh before Yan Gomes hit a single to center field. Gomes was replaced by speedster Rajai Davis, who stole second base, his 44th stolen base of the season, to get into scoring position with Vizquel at the plate.
Vizquel drove a 2-1 curveball up the middle to give the Blue Jays a 1-0 lead.
"He was dominating today," Vizquel said of Lester. "We couldn't really get to him. We had very few opportunities. ... It was a good win overall."
Blue Jays rookie Adeiny Hechavarria took the very next pitch over the fence in left field for his second career homer, increasing Toronto's lead to 3-0.
Daniel Bard came in to work the eighth, but he struggled and was removed without recording an out, as Toronto went on to tack on two more to go up 5-0.
It took away from what was an otherwise dominating performance by Lester, who has turned a disappointing season around.
Lester retired 12 of the first 13 batters he faced, and before the seventh, only once did the Blue Jays get two runners on base against Lester in the same inning. The only hitter to reach base against him through four innings was Vizquel, who hit a double down the left-field line but was stranded on second, as Lester came back to retire three straight to escape the threat.
In the fifth, Lester ran into his first real challenge of the game, as the first two runners of the inning reached base against him. He led things off by issuing his first walk of the game, a free pass to Moises Sierra and then botched a potential double-play ball.
With Sierra at first, Yan Gomes laid down a sacrifice bunt that Lester fielded and threw into center field trying to nail Sierra at second base. But he didn't let it faze him, striking out the next two batters before getting a groundout to end the frame.
The left-hander, who retired the side in order in four of the first six innings, fell to 9-12 on the year.
Lester went seven innings, allowed three runs on four hits, walked one and struck out seven. He threw 68 of his 101 pitches for strikes.
"Great, awesome. Best I have thrown all year," Lester said about his outing.
Lester, who has allowed three runs or less in six of his last seven starts, recorded his 16th quality start of the season.
"I don't know if he threw a bad pitch except for maybe the curveball to Vizquel," manager Bobby Valentine said. "The whole game he was in and out, doing a heck of a job."
Valentine said prior to the game he felt Lester turned his season around when he was left in to absorb a career-high 11-run pounding to the Blue Jays back on July 22 in Boston.
Since then, the left-hander is 4-4 with a 4.03 ERA and has been more efficient by throwing more changeups and a harder curveball, according to Valentine.
Lester, who has a uncharacteristic 4.95 ERA on the season, has noticed the improvement as well.
"I have to keep looking at those positives and keep plugging away," Lester said.
Boston attempted to mount a comeback in the eighth, but Cody Ross hit into an inning-ending double play with runners at the corners.
In the seventh, Pedro Ciriaco hit a two-out single and then stole his 14th base of the season with Jose Iglesias at the dish. In the middle of the at-bat, Valentine used Daniel Nava as a pinch-hitter, but he grounded out to end the inning.
"It's not kindergarten here. You don't think it was a good decision? So what. I think it was the right thing to do to try to win the game for a guy [in Lester] who is busting his butt out there," Valentine said when asked about the rare move.
Iglesias said he felt good and wanted the opportunity to deliver, but he understood why Valentine went to Nava.
"He made his decision, he tried to do what's best for the team and I respect his decision," Iglesias said.
The Red Sox were unable to capitalize against Blue Jays starter Brandon Morrow after putting two on with one out in the first and then again in the fourth.
Morrow threw six shutout innings of four-hit ball, walked one and struck out three.
"We fired a lot of blanks with men on base today," Valentine said. "We had some chances. Morrow made some really good pitches on the outside part of the plate with his fastball. He did a good job to get out of it."
Boston will continue its seven-game road trip Monday night against the Rays.