There's a problem with that.
I've yet to come across a hard-and-fast rule that determines when a player is eligible, and when he is not. According to most sources, Gross was thought to be unavailable for the game yesterday, because he was technically no longer a member of the Brewers. If that is the case, then the Brewers may very well find themselves in the position of facing a possible forfeit.
Again, the rules seem very vague on all of this, but as far as I can tell, if a team uses a player who is not eligible to player in a game, then they should be forced to forfeit the game. A team forfeiting a game because of a player's eligibility is rare, but it happens. It happened last year in the Cardinals organization.
There are a bunch of other issues here, of course, chief among them exactly when the deal for Gross was finalized; there's definitely a chance that it wasn't completely done until after the game began. However, given that the media seemed to know that Gross had been dealt before yesterday's contest, that argument seems to be on a little shaky ground, to say the least.
What I find most interesting here is the tie between the Milwaukee franchise and the commissioner's office. Bud Selig founded and owned the Brewers for quite a long time; how he deals with a potential rule infraction like this could very well turn into a big story, one way or the other.
Hopefully, details will come relatively quickly; I hope to know whether there are going to be any ramifications to the outcome of the last series before the next one begins. More as, (or if) this story develops…