From Peter Abraham:

Hideki Matsui was unavailable for the third straight game with an inflamed left knee last night and could be placed on the disabled list today.

"It's always a possibility," manager Joe Girardi said before the Yankees had their game against the Pittsburgh Pirates rained out in the third inning. "You don't want it to come to that. But if we feel it's in the best interest of him and the club, that's something we'll talk about.

It doesn't sound too serious, so hopefully if he does hit the DL, it'll be retroactive and for a short time.