In Yahoo's game summary recap, at the bottom under "Notes":
Bay fouled a ball off his left knee in the fifth inning and departed in the sixth with a bruised knee. ``I'm fine,'' he said.
Well, it sounds like at least Bay himself doesn't think it's bad (so far). And there's no mention of X-Rays or MRIs yet, so maybe it's really no big deal -- maybe he'll just miss a day or two w/ a bad bruise.