The club confirmed Lukaku was a no-show after being called back to their training ground, sparking threats of disciplinary action in a saga that has left the former Chelsea and Inter Milan striker's immediate future hanging in the balance.
Lukaku withdrew from Belgium's trip to the United States, missing World Cup warm-up matches against the U.S. and Mexico to work on his fitness. Italian media reported that Lukaku refused to return to the club's training ground for rehabilitation.
"Napoli can confirm that Romelu Lukaku did not respond to today's call to return to training," Napoli said in a statement.
"The club reserves the right to consider taking the appropriate disciplinary action, as well as to determine whether the player will continue to train with the team for an indefinite period."
Lukaku, who scored 14 goals last season to guide Napoli to the Serie A title, has endured a torrid campaign plagued by fitness issues.
He was sidelined for the first half of the season due to a hamstring injury while he has managed one league goal in five substitute appearances since his return in January.
Lukaku defends decision to stay in Belgium
The 32-year-old striker defended his decision to remain in Belgium for treatment, citing ongoing physical problems that have hampered his return to peak form.
"The truth is the last few weeks I wasn't feeling right physically and I had it checked out while I was in Belgium and it showed that there was an inflammation and liquid on my hip flexor muscle next to my scar tissue," Lukaku said on Monday.
"Since it is the second problem I had since coming back early November, I chose to do the rehab in Belgium so I can help when called upon.
"I could never turn my back on Napoli, never. There's nothing more I would love to do than play and win (with) my team. But right now I have to make sure I am clinically a 100%."