Romelu Lukaku will miss Napoli's opening fixture of their Serie A title defence after suffering a thigh injury which could keep the Belgium striker out for months, the Italian champions said on Monday.

Napoli, who travel to promoted Sassuolo on Saturday, said in a statement that Lukaku had suffered a "serious" tear in his left thigh during Thursday's pre-season friendly with Olympiacos.

No indication over how long Lukaku would be sidelined was given by Napoli, but Italian media reported that he could be out of action until November.

Napoli suggested that Lukaku, 32, might have to undergo surgery.

Lukaku was key to Napoli winning their fourth league title last season, setting up 10 Serie A goals to go with the 14 he scored under coach Antonio Conte.