Napoli have confirmed their squad for the Europa League round of 32, and it has remained unchanged except for their transfer window departures. Since none of those players spent any time on the pitch this season, Napoli will face La Liga side Villarreal with the same squad that won all six of their Group Stage matches.
Neither Vasco Regini nor Alberto Grassi, Napoli's two January signings currently in the first team, were named to the squad. Regini is cap-tied in the Europa League after he played for Sampdoria in their failed effort to qualify for the competition, so he's unavailable. Grassi's injury makes it pointless for them to register him right away, especially since Napoli will be able to adjust their squad later on in the competition. That gives them the chance to get him healthy and evaluate his effectiveness before committing him to the Europa squad.
Some outlets have reported that Grassi is on Napoli's "B" list -- a sub-list of under-21 players who are available to be played -- but two things conflict with that. First, as a brand-spanking-new-signing, he doesn't qualify for the B list by UEFA's published rules:
A player may be registered on List B if he is born on or after 1 January 1994 and has been eligible to play for the club concerned for any uninterrupted period of two years since his 15th birthday by the time he is registered with UEFA. Players aged 16 may be registered on List B if they have been registered with the participating club for the previous two years without interruption.
Secondly, Grassi's name is conspicuously absent from the Napoli squad list on UEFA's website, which has already been updated to reflect roster changes across the competition. That would certainly suggest that Grassi is not on the B list.
For their part, Villarreal only made a single change to their squad, dropping loaned-away defender Bojan Jokic in favor of young midfielder Alfonso Pedraza, who plays for Villarreal's B team in Spain's third tier but hasn't been with the club long enough to qualify for the B list.