Rotoworld's source for the DL speculation is a twitter post by MLB.com's Jordan Bastian: "X-rays negative on Downs' left foot. MRI tomorrow. Seems like a likely DL case."
I've never seen twitter used for this before, personally, and it's pretty amusing. Obviously, not much detail is offered as to why he thinks the DL is likely.
There's this from Fanhouse:
An X-ray taken Tuesday was negative, but Downs is scheduled for an MRI on Wednesday, and the way he was limping after the game, the disabled list seems like a strong possibility.
http://mlb.fanhouse.com/2009/06/16/cito ... gue-games/
Again the speculation seems to center on the fact that Downs was limping. So not much to go on yet.