It needs to be Lawler, especially now. Nick has seen him get KO'd, ironically for the first time since Nick himself KO'd him to 12 yrs ago; he will be aggressive. It's the perfect fight to bring out the best Nick Diaz. Robbie isn't a long and tall rangy striker with great movement. He's a natural WW, and he's probably the best brawler ever.
Robbie has been KO'd and lost his belt. He probably just wants immediate redemption. Since the ideal fight is off the table (Woodley), he'll gladly accept a huge fight (a "money fight") against the only other guy to KO him.
Welcome back, Nick. Welcome back, Motherfucker.