On a day when Boston needed its ace left-hander to play stopper, he couldn't even make it out of the third inning.
Derek Jeter belted a three-run homer to cap a six-run second, and New York scored eight times against Lester in just 2 2/3 innings Saturday afternoon, delivering the wild card-leading Red Sox yet another blow with a 9-1 victory.
The Red Sox had their lead trimmed to 1 1/2 games over the Tampa Bay Rays, who beat Toronto 6-2 on Saturday night to narrow the gap by a full game.
Lester (15-9) wasn't able to press pause on Boston's epic September slide, losing his third straight start after giving up eight hits and a walk in his shortest outing of the season.
The Red Sox lost for the sixth time in seven games and fell to 5-17 in September.
They will finish their rain-delayed series with a doubleheader Sunday at Yankee Stadium.
"If we come out tomorrow and win the two games, we'll pretty much be a lock to be in the playoffs," said Red Sox first baseman Adrian Gonzalez. "Things can turn in a matter of, what, 15 hours?"
Their three-week fall has been so precipitous that the Red Sox, who led the AL East by 1 1/2 games entering the month, are now one game under .500 (33-34) since the All-Star break.
"We all want to win. It's up to us to go do it," said Red Sox manager Terry Francona. "We know what's in front of us, we just have to play better."
Meanwhile, the Yankees clinched homefield advantage throughout the AL playoffs.
Their promising 21-year-old rookie, Jesus Montero, had a solo homer among his three hits and knocked in four runs while Freddy Garcia (12-8) threw six scoreless innings for his first win in four starts.
Boston's only run came on Carl Crawford's RBI double off Raul Valdes in the seventh inning, which scored Conor Jackson after he singled.
But the six batters in between weaved a nightmare for the Red Sox left-hander, culminating in Jeter's three-run blast for a 6-0 lead.
Robinson Cano and Andruw Jones both singled around a Nick Swisher walk to load the bases for Montero, whose base hit scored the first run. Russell Martin followed with a two-run single and Jeter homered on the very next pitch, reaching out for a fastball and lining it into the right-center field seats.
Despite retiring the first two hitters, Lester couldn't make it out of the third inning. Swisher and Jones both singled and Montero drove a ball to the wall in left-center, scoring both runners for an 8-0 lead. Scott Atchison then got the call and induced a fly out from Martin to end the frame.
"We beat him today," Jeter said of Lester. "But I'm not concerned with what I see in him. We're trying to win our games and stay sharp."
Montero led off the sixth inning with his fourth homer of the season, a blast to right field off Junichi Tazawa that made it 9-0.
Lester threw 55 pitches, including 32 for strikes, and fanned three batters. He is 0-3 in three starts since going 4-0 in his previous five...The Yankees improved to 5-1 on an eight-game homestand. They wrapped up their 17th AL East title on Wednesday.