Napoli have confirmed Lorenzo Insigne suffered a right adductor strain against Manchester City in the Champions League on Tuesday.
The Italy international came off as a precaution in the 57th minute of the 2-1 loss at the Etihad Stadium.
According to a statement on the Partenopei's website, Insigne underwent tests that have determined the nature of the problem.
The 26-year-old has begun treatment but it is not known when he will return to first-team action.
Maurizio Sarri's Serie A leaders are due to face second-placed Inter at Stadio San Paolo on Saturday.