As first reported by Standard Sport a year ago, Lukaku has made a return to Chelsea his priority because after admitting to having 'unfinished business' at Stamford Bridge.
The Belgium international has refused a new contract at Goodison Park, and Everton will reportedly demand £100million for the Premier League’s top scorer.
Koeman believes that Lukaku is a better all-round player this season, irrespective of the 24 goals which have made him the favourite to land this season's Golden Boot.
Lukaku is already six better off than last year's top-flight total and while that has helped Everton secure Europa League football, it has made him even more of a target for potential suitors.
"He has scored 24 goals and with five games to go he can reach a really high total," said Koeman, who refused to speculate on suggestions of the striker's reported £100m price tag.
"It is like for any player who is performing at a high level - every team likes to have goalscorers in the team because goals make points for the team.
"But I think he is improving in the rest of his football. He is doing better pressing and running in the channels.
"He is now a better striker for the team than he was last year.
"I've had several conversations about how, in my opinion, he needs to play as a striker and he is working hard and improving."
The relationship between Everton and Chelsea soured after Roman Abramovich's club had four bids rejected for defender John Stones, who is now at Manchester City, in 2015.
But Chelsea’s bargaining position has been strengthened with Everton now interested in a loan deal for their Academy starlet Tammy Abraham, who has impressed on loan at Bristol City.
Koeman added of any potential interest in Lukaku: "If someone is interested for sure they will call the board and then we will speak about it but we will try to do everything to keep him at Everton next season."
Additional reporting by the Press Association.