Napoli threatened Romelu Lukaku with disciplinary action on Tuesday after he decided to stay in Belgium for treatment on a hip injury.
“Romelu Lukaku has not responded to our call to return to training,” said Napoli in a statement.
“The club reserves the right to take appropriate disciplinary measures, as well as to review the player’s continued involvement with the squad.”
Lukaku, who has had a season severely disrupted by injuries, withdrew last Tuesday from Belgium’s World Cup warm-up friendlies against tournament co-hosts Mexico and the United States.
The centre-forward explained on Monday that he had inflammation and liquid on his hip flexor muscle.
“I chose to do the rehab in Belgium... (But) I could never turn my back on Napoli, never,” said Lukaku.
Having suffered a hamstring injury during pre-season, Lukaku did not return to competitive action until January this year.
He has made just seven appearances for the Serie A champions in all competitions this term as they have endured a tough title defence -- Napoli currently trail league leaders Inter Milan by seven points.
Napoli host second-placed AC Milan on Easter Monday hoping to leapfrog the seven-time European champions in the league standings.
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