The most likely scenario (given he doesn't wrestle) is that someone attacks him off-screen and lays him out. As a result, Paul Heyman turns to Curtis Axel as the replacement, who ends up getting a big pin fall victory over Jericho (with some sort of cheating, I'm sure.) The next night, Heyman says that he wants Axel to find out who attacked Punk and then do the same to them. Axel says there's no need to look because he's standing right here. He confesses to laying out Punk because he feels Heyman is making a mistake with relying on Punk to fight a match with Jericho instead of him.
Heyman is impressed with the balls on Axel to go after the "best in the world" and take him out. He continues to support Axel over the coming weeks until eventually, Punk returns and some serious shit starts up, likely resulting rather quickly with Punk round house kicking Heyman in the skull and going after Curtis Axel for revenge.
I know Chicago fans will be pissed (and fans in general) of not getting to see Punk wrestle after being "advertised", but they'll simply try and transfer that heat (storyline wise) to Curtis Axel for attacking Punk and denying the fans and especially the home crowd Chicago of seeing Punk. They'll likely even promote the match with a question mark by having Punk remain silent during the build to the PPV, leading people to speculate on their own that Punk might not wrestle so that they won't set themselves up for disappointment if he doesn't.