The 23-year-old continues to impress at Goodison Park, finding the net 24 times this season alone in the Premier League to top the division's scoring charts.
Lukaku has frequently indicated that he wants to play Champions League football, however, and will therefore not commit to fresh terms beyond the end of his current two-year deal.
Reports in the British press this week suggested that Lukaku could be the target of a £100m tug-of-war between Manchester United and Chelsea, but Koeman has again made it clear that he will not be bullied into selling his star man.
"I am not involved in that business. He has a two-year contract," he told reporters. "If somebody is interested [in him] for sure they will call the board of the club and speak about it, but we like to keep him at Everton for next season.
"He knows, I had several conversations about how in my opinion he needs to play as a striker. He is working hard, he is improving and with 23 goals with five games to go, he can reach a really high total. He is improving and now at the moment a better striker for the team than he was last year."
Lukaku further fuelled speculation of a move to either Man United or Chelsea by admitting this week that he subscribes to both sides' in-house TV stations.