The 23-year-old did not travel with the squad to their warm-weather training camp in Dubai due to a calf injury, instead returning to his home country of Belgium to undergo treatment.
However, Koeman is expecting to have the striker back available early next week and is confident that he will not miss any action with the problem.
"We know that for 10 days Rom has had some problems with his calf. He has a lot of confidence in the Belgian doctor and I gave him permission to go to Belgium and recover," Koeman told evertontv.
"We expect him, normally, next Tuesday to be joining in the training session. He will be fit and available for the weekend."
"They are progressing well. We knew that they would not play in the friendly this Friday but next week, from Tuesday on, they will be available for all the training sessions. Normally they would be fit for the weekend of Sunderland," he added.
Everton are currently on an eight-match unbeaten streak in the Premier League, stretching back to before Christmas.