Details of the injury are as of yet unknown, so it's not clear if this is a new injury or an aggravation of the one that kept him out on Wednesday. Either way, it's expected to keep him out between 10 and 15 days, which will keep the frequently-used midfielder out for several matches.
With that 15-day projection, Gargano could miss as many as four matches, including both legs of the Europa League round of 16 tie against Dynamo Moscow, as well as matches against Inter Milan and Hellas Verona. It's a tricky spell for him to miss thanks to Napoli's relatively thin midfield at the moment.
Hopefully the injury is nothing major and he returns to full effectiveness soon. As up-and-down as his form has been, Gargano is still needed.