He's already taking the fight on less than a months notice, that means he'll only have about 3 weeks to really train (with the final week spent cutting weight), and with a fever that'll either cut down on his training camp by a week, meaning he'll only have two weeks to train, or he'll start out his training camp feeling shitty and, thus, training and performing shittily. He knows Peralta's a dangerous fighter and, even though Peralta's taking the fight on a months notice as well, he's dangerous enough where he'd wanna train as much as possible and as best as possible, and with a fever it would essentially mean that by taking the fight he's going to fight with a significant disadvantage in either time spent training or training quality, or both.
He's also at the stage of his career where it's either the UFC or bust if he wants to make a comfortable living off of fighting professionally (the only other thing is winning a Bellator tourney, but that's not gonna be any easier than being in the UFC), and one loss in the UFC at this point would potentially mean he'd get cut, and how many guys that didn't win TUF that get cut end up making a comeback to the UFC, or have a respectable career afterwards at all? There's, like, Vinny Magalhaes, and that's one out of the hundreds of fighters that've been on TUF.
I get why he's backed out, he's not a pussy for it.