Lukaku moved to Goodison Park from Chelsea for the 2013-14 season on the final day of the transfer window, and has gone on to score eight goals in nine Premier League games for Roberto Martinez's side.
The Belgium international's latest strike came in Everton's 4-0 rout of Stoke City on Saturday, and he hinted after the game that he would consider staying on Merseyside beyond the end of the campaign.
This came after Lukaku intimated that he had been forced out of Stamford Bridge - a claim dismissed by Mourinho, who labelled the 20-year-old a "young boy who likes to speak".
The Portuguese indicated on Tuesday that he is in complete control when it comes to player arrivals and departures from Chelsea.
"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," he stated.
Mourinho, whose side head to Sunderland on Wednesday sitting four points behind Premier League leaders Arsenal, went on to express his confidence that Chelsea can overtake their London rivals at the top of the table.
However, he added that the same may not be true of fellow title challengers Manchester United and Tottenham.
"They (Manchester United) are nine points behind Arsenal - it's a gap, nine points is a gap," said the ex-Real Madrid boss.
"Tottenham have 10 (points) though, no? It's a gap. It's not a gap for us because we are speaking about five, six points. It's not a gap; it's something that in two matches is over."
