The timing of the news is more than a bit unfortunate -- most of Napoli's players are only just coming back from international duty, so it further disrupts an already-abnormal training routing with the club right now. A calf injury is also a tough one for a goalkeeper to deal with, because of the quick-twitch pushoffs and adjustments they need to make that heavily utilize the muscle.
Rafael Cabral and Gabriel were both mobilized into a special extra goalkeeping training session after the news, which is a smart plan for Napoli under the circumstances. Neither goalkeeper has gotten significant minutes this season or has played in months, and if they're going to be called into action they'll need all the extra training they can get in order to start building any kind of match sharpness.
Hopefully Reina recovers in time to be able to play because having him between the sticks gives Napoli a better chance of winning. Still, if his injury is still at all an issue come Sunday, the team should go with one of their backups, because risking making that injury worse and taking it from a short-term issue to a long-term one is something Napoli absolutely cannot afford right now.
Get some rest and heal quick, Reina. Napoli need you.