We already heard that Lorenzo Insigne's injury was much less severe than had been feared, but Monday brings even better news to the Napoli fanbase:
Insigne underwent an MRI scan for the knee injury that forced him off in the match against Juventus. The tests have ruled out damage. Lorenzo worked in the gym, and tomorrow will join the rest of the squad.
-Source: SSC Napoli official site
Hearing that Insigne will be back in full training so quickly is a huge dose of good news for Napoli. He'll probably still miss Thursday's Europa League match against Legia Warsaw in Poland -- a match he was likely to skip anyways -- but getting back in training so quickly means that Insigne will likely be available for the tilt against AC Milan on Sunday.
So if you've still been worried about Lorenzo at all, worry no more. It sounds like he's fine, really and truly, and should be back in the lineup very soon. Hopefully his form doesn't miss a beat, because he's been excellent for Napoli so far this season.