The Belgium international played in Saturday’s 0-0 draw with Middlesbrough last weekend but he was unable to travel with the squad for their mid-season trip to Dubai.
While the squad jetted off, the 23-year-old travelled for treatment in his homeland as he continues to struggle with a calf problem.
The news came as a shock to some of the club’s fans but Koeman is adamant he will be fit enough to take on Sunderland in their next match on February 25.
He told Evertontv: “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. We expect him, normally, next Tuesday to be joining in the training session. He will be fit and available for the weekend.”
He added: “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.”
Watford boss Javi Gracia does not think Abdoulaye Doucoure's form will be affected by his omission from France's latest squad.