Fatigue? They lost because they approached the first half (especially) terribly and their defence and reina gave away gifts.
There certainly were individual mistakes, but fatigue was an issue too. Sarri said before the game that the team is a bit fatigued currently. In the game, Napoli players were too slow to close down spaces which is uncharacteristic of them. All of the midfielders played poorly, particularly Hamsik. That's understandable because Hamsik and Allan barely got a breather all season. Insigne and Callejon up front didn't get much rest either lately because Mertens was injured.
Every time Bologna pressed and attacked, Napoli looked in trouble. In the 2nd half, Bologna started off parking the bus but were getting slaughtered (except for poor finishing). Bologna started pressing and attacking again, and Napoli looked in trouble again. They were getting beaten during transitions. Aside from individual deficiencies at the back, fatigue was also an issue.
They had a great run prior to this loss. One loss after a while isn't that big an issue for them. Main worry for them should be not having a decent backup for Allan and Hamsik. If ADL is smart, he'll change his mind about not signing anyone in Jan because, otherwise, Allan and Hamsik will get burned out.
As for us, like I said, the game vs Fiorentina will tell us how good we are. We've improved and have had a decent run, but the quality of the opponents weren't too high. For me, a top-3 spot was never in doubt, but I'm not sure if we're scudetto favorites yet. Will have to wait and find out.