Lukaku turned down the option to extend his contract at Goodison Park earlier this year and now has just over two years remaining on his current deal, leading to speculation that he could leave the club this summer.
The 24-year-old was the Premier League's top scorer before Harry Kane overtook him with his four-goal haul against Leicester City on Thursday evening, and Koeman acknowledged that he needs to look at sharing the goalscoring mantle more evenly next season.
"Even if he doesn't win the golden boot, 24 goals is a good number," he told reporters.
"He can improve - he knows that by himself. He is just 24 years old and he can improve, and needs to in several aspects, as a nine, as a target man - to keep the ball better, be stronger and do better pressing in front. But that is what we work on to improve.
"What we need is to keep his goals and find more players to score more than four or five in one season."
Lukaku has failed to score in his last four games, having netted 12 times in his previous 10 outings.