Cashman said that Matsui had the procedure performed on Thursday, when the Yankees opened their new stadium with a 10-2 loss to the Cleveland Indians.
The next day, Yankees manager Joe Girardi said that Matsui may have taken "a little step back" in his return from left knee surgery last September. Matsui is hitting .194 (6-for-31) with one home run and three RBIs in 11 games for New York this season.
"During Spring Training, fluid had been coming and going," Matsui said through an interpreter. "It's been a repetition of that. I don't really look at it as a setback. I think it can be controlled to a certain extent with treatment and conditioning."
Matsui said that the knee affects him most when running, obviously, but that it is possible it has also impaired his ability to hit. Girardi said that Matsui's knees are a subject that will have to be watched closely, as his availability may be in question.
"We're hoping that we can get him back," Girardi said. "I've said all along he was probably my biggest concern over the course of the year. His knees are pretty beat up, and you don't know how they're going to react from day to day."
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.