Before the UFC 140 post-fight press conference a rumor started to circulate among media that the UFC was about to announce that Alistair Overeem was out of his UFC 141 main event with Brock Lesnar. The popular rumor was that Frank Mir, on the strength of his arm breaking win over Antonio Rodrigo Nogueira, would replace Overeem for a rubber match with Lesnar.
While Dana White was quick to shoot down the rumors, asking where they come from, the rumors did persist. And White denying that the fight was in jeopardy doesn't mean that there aren't issues. White wouldn't talk about this in public if things weren't clear one way or the other. Then again, it could all be a rumor and mean nothing.
As for the reasons for a possible issue, Josh Gross from ESPN tweeted a very interesting bit of information:
Now, obviously this isn't based on anything more than hearsay and I'm certainly not saying that there was an issue with Overeem's pre-fight test. I'm just passing along information on what is being talked about by the media. He is on the NSAC agenda so that means there may be some sort of issue, but we'll have to wait until Monday to get the full story.
Until then, again, it's all just rumor.