Over four-plus innings, he gave up 11 earned runs and four homers, both career highs. Lester (5-8, 5.46 ERA) equaled his previous high in walks with five while giving up nine walks and striking out just two.
Lester has turned in just one quality start in his last five outings.
"It's embarrassing," Lester said. "I've let my team down a lot this year. It's hard for me to walk around this clubhouse and look guys in the eye right now. I'm not pitching well. I'm not doing my job. Guys scored seven runs today; we should win that game. Like I said, it's embarrassing. That's all I can really say about it."
This was the first time a Red Sox starting pitcher has allowed 11 earned runs since Doug Bird on May 5, 1983.
"That's not a typical day for Jon Lester by any means," said Blue Jays manager John Farrell, who was Lester's pitching coach for four years in Boston. "He's a [darn] good pitcher, a talented one. As we've seen from our own guys, they go through stretches where things aren't clicking. It just goes to show you that the elite in this game ride a little bit of a fine line when their performances are consistent and well-above average to days when they scuffle, and that was one today."
As they embark on a six-game road trip against the two best teams in the American League -- the Rangers and Yankees -- the Red Sox are again a .500 team at 48-48.
"We've got to play well, especially at this time in the season," said second baseman Dustin Pedroia. "We need to find a way to grind out wins. We're trying to find the identity of our team. We want to do something special."
The Red Sox had some hope coming out of the All-Star break, winning five of their first seven. But they got swept in three straight by the Blue Jays to cap the homestand with a 3-4 record.
For the first time since June 19, Boston is back in last place in the American League East. The Red Sox are 3 1/2 games back in their pursuit of a Wild Card berth.
"We've just got to figure out a way to get that [momentum] going," said outfielder Cody Ross. "We have to forget about this and then start off again tomorrow and just keep grinding. I know I keep saying that a lot, but that's what we have to do."
Without question, the biggest problem plaguing the team at the moment is the underperformance of Lester and Josh Beckett, who were projected to be the team's top two starting pitchers. In games started by Lester and Beckett, the Red Sox are 13-23.
"It's never easy. It's particularly hard when you're struggling," said Beckett. "I think sometimes you can get information overload from too many people, and I'm not saying that's exactly what's going on [with Lester]. I don't know the situation, but sometimes I think it's nice to kind of remove yourself. Obviously you want to work to get better, but sometimes you have to go backwards to go forward."
The one common thread between the struggles of Lester and Beckett is that both have struggled all too frequently in the first inning.
"I've never seen an offense put in such positions as our offense has been put in, battling from the start of the game," said Red Sox manager Bobby Valentine. "And I'm proud of them. They keep battling back, keep, you know, scratching away and fighting, but it's not fair."
For Lester, the day was bad from the very start. His first pitch of the game was hammered over the Green Monster by Brett Lawrie for a solo homer.
From there, the hits kept coming. By the time the top of the first ended, Lester and the Red Sox were in a 5-0 hole.
What could be the cause of Lester's struggles?
"The one thing that through our reports and through seeing him earlier in the year, there has been a little bit more of a tendency to miss over the plate and miss up in the strike zone," said Farrell. "The devastating cutter that he has had for so long might not be as sharp right now, so it gains a little length in the break to it. So I think our hitters are able to see it a little bit earlier in the flight to home plate. We laid off some pitches, but we didn't miss many when they found their way into the middle of the plate."
Lester's confidence at the moment is about where you'd expect.
"It's obviously not at the highest it's ever been. The thing is, nobody's going to feel sorry for me," Lester said. "I've got to go out and pitch, and I've got to pitch better. I'm not worried about my confidence, I'm not worried about my mechanics. I'm not worried about anything except trying to execute pitches. And I'm not doing that."
Despite Lester's shaky opening, the Red Sox showed quickly that they weren't deflated. With one out, Adrian Gonzalez crushed a three-run homer into the bleachers in right-center.
Though his offense got him right back into the game, Lester couldn't keep the momentum. He opened the second with a walk to Lawrie, and with two outs, also walked Edwin Encarnacion. That led to some bad developments. J.P. Arencibia unloaded for a three-run homer and Rajai Davis followed with a solo shot. Suddenly, the Red Sox were down, 9-3.
"I want him to get better," Valentine said. "He's a great pitcher, a great guy and I feel this as much as he does and I know he's taking it tough."
Thanks to a pair of inning-ending double plays, Lester settled down briefly. But his outing ended abruptly in the fifth when Davis led off with a walk and Travis Snider followed with a two-run homer to center, putting Toronto ahead, 11-4.
Before Lester walked off the mound, Pedroia gave him some words of encouragement.
"We have his back. Everyone goes through tough times," Pedroia said. "He's not going anywhere. He's our horse. We love him. I'll play behind him any day of the week. I just wanted to make sure he knew that, because everyone goes through tough times. It's good to have reassurance from your teammates that we have your back."
Jacoby Ellsbury hit his first homer of the season in the fifth, a solo shot. And Boston scored two more in the sixth to make it 11-7.
"We tried to come back again today," Valentine said. "If you keep that spirit, you're going to win a lot of ballgames."
More than spirit, however, the Red Sox need their aces to start pitching like aces again.
"God, I hope so," Lester said. "It's getting old. [I'm] not a big fan of [pitching poorly], to put it lightly."