This move has been expected for several days, with his medical actually taking place on Friday ahead of Monday's announcement. Napoli also confirmed in their announcement that they hold an option to purchase Regini outright at the end of the season, a clause believed to be valued around €1.8 million.
Regini, 25, has mostly played as a left back for the last year and a half at Sampdoria, but he has more experience playing in central defense. His versatility and left-footedness gives Napoli something they've been looking for on the back line -- they previously had no left-footed central defenders with experience -- and getting that skillset so cheaply is solid business.
Don't expect Regini to step into the lineup, though -- Napoli bought him to be a backup and rotational option. We'll probably see plenty of him coming off the bench and coming into the lineup against lower-table teams as needed to give Kalidou Koulibaly or Raul Albiol a break, but Regini won't be a regular starter unless someone gets hurt -- and even then it seems likely that Vlad Chiriches would start most of the time in front of Regini.
Still, this is a solid signing, and there's nothing to be disappointed about here. Regini is a good player, and fills a need in the squad perfectly. Hard to argue with that. Welcome to Napoli, Vasco.