Winless in eight starts since beating Boston on Aug. 6, Chamberlain responded with his first victory in 1 1/2 months and led the Yankees over the Red Sox 9-5 Friday night.
Earlier this week in Anaheim, Calif., manager Joe Girardi and Yankees coaches met with the 23-year-old right-hander.
"I wouldn't say that we read him the riot act. We just had a discussion that we knew he was capable of pitching better and that we need to see him pitch better," Girardi said.
Chamberlain (9-6), slated to be the Yankees' No. 4 starter in the AL championship series and World Series, had scuffled while the Yankees held down his pitch count and innings to keep him under his preset and undisclosed limits. He had gone 0-4 with 8.42 ERA in eight starts since beating Boston.
On Friday, he retired his first 11 batters before Victor Martinez's solo homer and allowed three runs and five hits in six innings, throwing 86 pitches. He struck out five and walked one.
"You've just got to be challenged," Chamberlain said. "There's a lot of people in this game that want your job and you know when it comes down to it you have to look like they're going to try to take money off your table and try to take away wins and a championship away from you."
Alex Rodriguez homered, drove in four runs and tied a career high with three steals as the Yankees, already assured of a playoff berth, lowered to three the magic number for their first AL East title since 2006.
Jon Lester (14-8), Boston's best starter during the second half of the season, crumpled to the mound in pain when struck near his right knee by Melky Cabrera's liner in the third inning. His head back and eyes closed, Lester was looked over by trainers from both teams. After about 2 minutes, Lester hobbled off with the assistance of Red Sox trainer Paul Lessard, and David Ortiz helped him down the dugout steps.
"When it first happened, it looked terrible. It sounded terrible," Red Sox manager Terry Francona said
X-rays were negative, and Boston said Lester had a bruised quadriceps.
"It hit off my knee going about mach seven," Lester said. "It hit me in a good spot, where it didn't do too much damage that, hopefully, I won't miss any time."
He allowed five runs and eight hits in 2 1/3 innings.
"I had good stuff tonight. It was one of those nights they made it seem like I didn't," Lester said.
New York has won seven of eight against Boston, including a four-game sweep at Yankee Stadium from Aug. 6-9, after starting the season 0-8 against the Red Sox.
Rodriguez's two-run homer in the third was New York's team-record 127th at new Yankee Stadium. New York hit 126 homers at old Yankee Stadium in both 2004 and 2005, according to STATS LLC.
The Yankees, who led 5-0 after three innings, also swiped seven bases in seven tries off Jason Varitek, their most steals since they had eight at Oakland on June 2, 1996.
"We wanted to be aggressive today, get runs early," Yankees captain Derek Jeter said. "Like I said in Anaheim, we're in the playoffs, but we haven't accomplished what we want."
Ortiz added a two-run drive off Chamberlain and had three RBIs. Ortiz has 27 homers, all but one from June 6 on.
With New York 6 1/2 games in front of the Red Sox, and Boston seven games ahead of Texas in the wild-card race, there is little drama in the three-game series. New York would clinch the division with a sweep, and Boston has a magic number of three for the wild card.
"If we're fortunate enough to keep playing, when this is over, nobody's going to care what happened," Francona said.
The Yankees plan to start A.J. Burnett, Chad Gaudin and Chamberlain against Kansas City next week. That would enable them to flip flop Burnett and Andy Pettitte, with Burnett following CC Sabathia and starting Game 2 of the playoffs. ... Francona intended to start Varitek in two of three games over the weekend, but might start Martinez behind the plate when Daisuke Matsuzaka pitches Saturday. ... A-Rod has 93 RBIs. ... Martinez extended his hitting streak to 24 games. ... Panama President Ricardo Martinelli threw out the ceremonial first pitch. Wearing a No. 42 jersey of Mariano Rivera, he bounced the pitch to the Yankees' Panamanian closer. ... The crowd of 48,449 was just the sixth sellout in 76 games at new Yankee Stadium -- the others were on opening day and during the August sweep of the Red Sox.