NEW YORK — The Yankees added a fresh arm to their bullpen today, purchasing right-hander Dave Robertson's contract from Triple-A Scranton/Wilkes-Barre. New York also optioned lefty Kei Igawa back to Triple-A, one day after he was called up.

In other news, the Yankees acknowledged that Hideki Matsui could need surgery on his ailing left knee if it doesn't start feeling better. The slugger was placed on the 15-day disabled list Friday.

"Hopefully, this is a 15-day thing and we get him healthy and get him better," manager Joe Girardi said. "Could it be more? Yes."

Matsui had surgery on his right knee last offseason, but his left one is hurting now. The designated hitter and part-time left fielder is batting .323 with seven homers and 34 RBIs in 69 games.

"It's sore. There's some issues in there, obviously," Girardi said. "He had to have it drained again yesterday and hopefully it calms down now."

Asked if surgery was a possibility, Girardi said: "I think when things don't get healthy, surgery is always your next bet. But we're hoping to avoid that."

General manager Brian Cashman said surgery for Matsui would be a "last resort."

The 23-year-old Robertson was 3-0 with a 1.64 ERA and one save in 20 relief appearances for Scranton. He had 48 strikeouts and 16 walks in 33 innings.

"I figured sooner or later I'd get a shot, but I got the call sooner than I expected," said Robertson, who began the season at Double-A Trenton. "All I was trying to do was to get here."

Robertson was picked by the Yankees in the 17th round of the 2006 draft out of Alabama.

Girardi also said Sidney Ponson will get another start, on Wednesday night against his former team, the Texas Rangers. Dan Giese is headed back to the bullpen, but was not available Saturday against the Mets after starting the opener of Friday's two-stadium doubleheader.

With a history of off-the-field problems, Ponson pitched well for Texas this season but was cut by the Rangers for being a disruption.

He signed a minor league contract with the Yankees and was called up Friday night to start Game 2 of the Subway Series doubleheader. Ponson was a huge success, tossing six effective innings in a 9-0 victory. Igawa worked the ninth to finish it.

To make room for Robertson on the 40-man roster, the Yankees designated minor league right-hander Oneli Perez for assignment. Perez was claimed off waivers from Cleveland on June 18.

Also, right-hander Ian Kennedy was to pitch for Class-A Tampa on Saturday. Kennedy was recently activated from the disabled list and optioned to Tampa.

Phil Hughes (fractured rib) is about three weeks from pitching in a minor league rehab game, Girardi said, and probably seven to eight weeks from returning to the Yankees.