The Belgium international leads the Premier League scoring charts with 24 goals in 32 appearances and has been named in the PFA Team of the Year for his performances.
Koeman stated this month that Lukaku told him he will not sign a new contract, prompting increasing speculation that he could be signed by a European powerhouse in the next transfer window.
Old club Chelsea, Manchester United and Bayern Munich are among those to have been linked with a bid of as much as $170 million for the 23-year-old.
Koeman is delighted with the way Lukaku has developed his game and says he hopes to have him at his disposal for 2017-2018.
"I think he's improving in the rest of his football, doing a better pressing, runs in the channel," Koeman told a news conference ahead of Saturday's match with West Ham. "He's a better striker for the team than he was last year.
"I'm not involved in that business [of transfer bids]. He has a two-year contract.
"If somebody is interested then for sure they will call the board of club and then we will speak about it. But we would like to try to do everything to keep him at Everton next season.
"I had several conversations about how, in my opinion, he needs to play as a striker. He's working hard and improving and with five games to go he can reach a really high total.
"Every club likes to have goalscorers in the team."
Reports in England on claimed that England international Michael Keane has told Burnley that he wants to leave at the end of the season.
Everton have been linked with the 24-year-old in the past but Koeman would not discuss "interesting" transfer topics at this stage of the campaign.
"I don't talk about interesting players," he said. "It's a really interesting process for players and we know we'll have competition with bigger clubs. It's always difficult."