Going over the entire active MW roster these are the only viable replacements:
Alan Belcher
Derek Brunson
Chris Camozzi
Francis Carmont
Rich Franklin
Ed Herman
Brian Houston
John Howard
Lorenz Larkin
Cung Le
Thales Leites
Ronny Markes
Costa Philippou
Yoel Romero
Ronaldo Souza
Brad Tavares
Brunson, Camozzi, Houston, Larkin, Markes & Romero are all possibilities because they're all on the Troops card already but none of them are main event names so they're extremely unlikey which leaves us with
Alan Belcher
Francis Carmont
Rich Franklin
Ed Herman
John Howard
Cung Le
Thales Leites
Costa Philippou
Ronaldo Souza
Brad Tavares
Belcher can theoretically be in a main event but he's lost 2 in a row so that's unlikely. Carmont makes a lot of sense ranking wise but is boring as fuck. Dana wants Rich to retire but Rich still wants to fight. Not sure he could make it son short notice though. Herman already has a fight booked but as we saw with Machida being pulled, he can still fill in and he does have some name value. Howards fight with Hall really sucked and he's just getting back in the UFC. I don't see him being thrown in a main event so quickly. Cung Le is supposed to be doing TUF China soon but maybe he can do them a favor by taking the fight. Leites has name value and is coming off of a great win over Watson. He'd make sense. Costa just lost that snoozer against Carmont. A Jacare rematch makes sense as well but Jacare got hurt against Okami and I don't know the extent of the injury. Tavares has won 4 in a row but hasn't exactly made waves and is not ready for a main event imo.
So, best possible choices imo would be
Rich Franklin
Ed Herman
Cung Le
Thales Leites
Ronaldo Souza
I'm gonna guess Thales gets the fight but would love to see either Franklin or Le take the fight