From the comments
As someone who fights semi-professionally, I'd like to second Mr. Neiwert's statement. The injuries that are described would be almost impossible to fake, especially the broken tooth. Let me put it this way: I have never in a sparring match received the severity of injuries that Mr. Mirecki received. Those types of injuries are consistent with a serious assault, not simply asking a friend to punch you a few times.Dave Neiwert pointed out the important fact that the hospital has obviously confirmed the basic physical evidence of an assault.
As for how the attack might have been coordinated, people seem to have forgotten the most obvious answer: they trailed him from his house. Just because Mr. Mirecki didn't notice the assailants' car until shortly before the assault does not mean that it wasn't there beforehand.
I feel foolish for having to defend the basically polite premise that a crime victim's account of a crime should be given credence until and unless contrary evidence emerges.