The striker is currently excelling on loan at Everton, and was reported to have said that he would consider a more permanent stay at Goodison Park if they were to achieve Champions League football.
However, Mourinho responded by subtly claiming that Lukaku's future lies in the hands of Chelsea.
"Lukaku said that he may stay with Everton?" he asked reporters ahead of their clash with Sunderland on Wednesday.
"Maybe there is a new rule in football, where when the season finishes, every player is free to decide his future.
"Maybe it's a new rule [that I have not heard about]."
The Portuguese boss recently claimed that Lukaku was withholding information about why he was sent out on loan in the summer.