Sexson was released by Seattle last week. The big first baseman slumped badly against right-handed pitching with the Mariners this season, but hit .344 with five home runs in 71 at-bats against lefties.

He came through right away for the Yankees, hitting a two-out RBI single in his first at-bat. Sexson finished 1-for-3 with a walk and a strikeout in New York's 7-1 victory over the Athletics.

"I think this lineup hasn't touched what it can do yet and I think if I can get in the middle of it, there's going to be a lot of opportunity for me to help out," Sexson said before the game. "It's exciting to come here and have an opportunity to at least push for a playoff spot down the stretch. It's going to be fun."

New York hopes Sexson can provide pop from the right side and spell left-handed hitting first baseman Jason Giambi. The Yankees' lineup is loaded with left-handed hitters, and New York has struggled against lefty pitching all year.

The Yankees also are missing two of their top hitters. Leadoff man Johnny Damon is on the disabled list because of an injured left shoulder and Hideki Matsui is sidelined with a swollen left knee that might require season-ending surgery.

Damon rejoined the team for workouts and could be activated next week. Matsui was examined by a doctor Friday and was told to rest his knee at least through this weekend.

On Monday, he will see Dr. Scott Rodeo, who performed arthroscopic surgery on Matsui's other knee last November.

"Then we will have a plan of attack, whether it is surgery or a more conservative approach," Yankees general manager Brian Cashman said. "Surgery is certainly moving up the list of options."

Matsui wants to get back on the field this season.

"It's not entirely my decision," he said through a translator.

The Yankees began the night six games behind first-place Boston in the AL East.

The 33-year-old Sexson hit .218 overall for last-place Seattle this season, with 11 homers, 30 RBIs and 76 strikeouts. He batted fifth Friday night in the opener of a three-game series against the A's.

"He's the type of guy who could change the game with one swing," Yankees manager Joe Girardi said. "We're taking it day by day. Hopefully, he can get hot for us."

To make room on the roster, the Yankees optioned left-hander Billy Traber to Triple-A Scranton/Wilkes-Barre and transferred right-hander Phil Hughes (fractured rib) from the 15-day to the 60-day disabled list.

Sexson and the Yankees reached a deal Thursday, and a person familiar with the contract said Sexson would be paid a prorated share of the $390,000 minimum salary. The person spoke on condition of anonymity because an official announcement had not been made yet.

New York picked up a fraction of Sexson's contract, leaving the Mariners to pay the rest of his $14 million salary.

A native of Brush Prairie, Wash., the 6-foot-8 Sexson became a frequent target of boobirds in Seattle the past two years. He hit a career-low .205 with 21 homers and 100 strikeouts in 2007, and often was benched in favor of Miguel Cairo this season.

Sexson has hit 30 or more homers six times, and has six seasons of at least 100 RBIs. He started his major league career in 1997 with Cleveland and later played for Milwaukee and Arizona.

Sexson signed a $50 million, four-year contract with the Mariners in December 2004. He topped 30 homers and 100 RBIs in his first two seasons, then dropped off significantly last year.

"The better you play, the more you'll play," Girardi said. "He has good power, plays good defense and we'll see what he can do."

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AP Baseball Writer Ronald Blum contributed to this report.