British media reported this week that Everton were asking a world record fee of £100million for the 23-year-old Belgian international, who has scored 24 league goals in this campaign.
"He's now a better striker for the team than he was at this time last year," Koeman told a news conference ahead of his team's trip to 13th-placed West Ham, who have landed only one victory in their last nine league games, on Saturday.
"If someone's interested they'll call the board of the club and they'll speak about it but we want to keep him. Every team wants goal scorers because they make points for the team."
Everton have lost only two league games in 2017 and are lying seventh in the league, level on points with sixth-placed Arsenal, although they have played two more games.
Koeman is targeting a top-five finish to secure guaranteed qualification to the Europa League next season and he hopes to remain in contention when Everton visit the Emirates in the last game of the season.
Read the full article on eurosport.co.uk: Koeman wants 'improved' top scorer Lukaku to stay at Everton