Napoli insist Dries Mertens was not injured when he left them to join up with Belgium's national team squad on Monday after he was ruled out for four to five weeks with a thigh injury.
Mertens, 28, has reportedly torn a thigh muscle while Belgium coach Marc Wilmots told Belgian television that the midfielder "felt discomfort as soon as he arrived from Napoli," adding that he underwent tests as a precaution.
Although still unconfirmed, those tests are reported to have revealed a tear rather than just a strain to Mertens' right thigh which is likely to rule the player out for more than a month, according to La Gazzetta dello Sport.
The same newspaper which also quotes Alfonso De Nicola, a member of Napoli's medical team, as saying "[Mertens] was healthy" when he left Naples.
Mertens played 21 minutes of Napoli's 1-0 win over Udinese at the weekend and left Italy without any complaints on Monday morning.
Wilmots could not say how long Mertens would be out for, but said it is going to be "beyond Friday and Tuesday next week," meaning he misses both of the Red Devils' friendly matches against Italy and Spain, both in Brussels.
Mertens limped out of Napoli's 0-0 draw at Genoa earlier this month with a muscular problem in the same area, although he was deemed fit enough to feature at the weekend.