"This is one of those things that I think will take a while to sink in," said Mussina, who has won at least 18 games five other times.
The playoff-bound Red Sox won the second game 4-3 on Jonathan Van Every's bases-loaded single with two outs in the 10th off Jose Veras (5-3). Devern Hansack (1-0) pitched the 10th for the win as Boston prevented the Yankees (89-73) from reaching 90 wins for the seventh straight season. New York finished five games back of the wild-card Red Sox and missed the postseason for the first time since 1993.
Less than three months before his 40th birthday, Mussina (20-9) gave up three hits in six shutout innings in his final start of the season. He returned to the field to bring out the lineup card for the second game.
Last year he had the highest ERA of his career (5.15). This season, it was 3.37 after he allowed one earned run in 16 innings over his last three starts.
"I'm proud of myself to be able to do this after last year," he said.
Previously, the oldest first-time 20-game winner was Jamie Moyer, who was 38 when he went 20-6 for Seattle in 2001. Mussina, who hasn't committed to playing next season, could be the first pitcher to retire following a 20-win season since Sandy Koufax.
"I just had a lot of fun playing this year," Mussina said. "I don't know what the future holds. When you've got 18 seasons in, it could always be your last year."
Mussina's previous high for wins came in 1995 and 1996, when he won 19 games each year for Baltimore. Mussina went 0-2 in his last four starts in 1996, leaving his final one with a 2-1 lead after eight innings only to watch Armando Benitez allow a tying homer to Toronto's Ed Sprague in the ninth.
With defending World Series champion Boston heading to the AL division series at the Los Angeles Angels starting Wednesday night and New York missing the postseason, the doubleheader lacked the usual intensity of Yankees-Red Sox matchups.
In the opener, after Boston closed to 3-2 in the eighth on Chris Carter's RBI grounder off Joba Chamberlain and Jacoby Ellsbury's run-scoring single against Damaso Marte, Rivera struck out Dustin Pedroia to end the inning. The Yankees added three runs in the ninth against Jonathan Papelbon, and Rivera finished with a hitless ninth for his 39th save in 40 chances.
Rivera returned from tests in New York and may have arthroscopic shoulder surgery.
"Joe told me a couple days ago he'd have everybody ready, and I guess that means Mo in the eighth," Mussina said. "I know he's not feeling 100 percent but he's still throwing the ball great. I'm glad he came back from New York."
Rivera got his 49th save in a game that Mussina won, trailing only the combination of Oakland's Bob Welch and Dennis Eckersley (57) and the Yankees' Andy Pettitte and Rivera (55).
"Guys were so happy," Yankees manager Joe Girardi said. "They were clapping when he came in and there were a lot of hugs."
Pedroia went 2-for-4 to increase his batting average to .326 but likely will finish second in the AL batting race to Minnesota's Joe Mauer, who was at .330. If the Twins play an AL Central tiebreaker on Tuesday, Mauer would have to go 0-for-7 for Pedroia to overtake him.
"I'm not real big into personal achievements like that," Pedroia said. "I've had a great season and if I finish second or third, or whatever it is, I'm happy with that. It's fine."
Xavier Nady hit a three-run homer in the fourth off Daisuke Matsuzaka (18-3), who lost for the first time in eight decisions since July 28, going four innings in his playoff tuneup.
"I didn't have any big goals today, but just to be able to throw in the game," Matsuzaka said through translator Masa Hoshino. "With how the weather has been, (that) was enough for me."
In a pregame ceremony, Johnny Pesky's No. 6 was retired by the Red Sox as the 89-year-old former shortstop stood under an umbrella at home plate wearing the team's white home uniform with his number on the back. The other five numbers retired by the team are Bobby Doerr (1), Joe Cronin (4), Carl Yastrzemski (8), Ted Williams (9) and Carlton Fisk (27).
