The Dutch demolished Estonia 5-0 on Tuesday, a game in which Koeman handed debuts to impressive AZ Alkmaar duo Calvin Stengs, 20, and 18-year-old Myron Boadu, who took his tally for the season to 17 goals in 24 starts for club and country.
"It would be very strange if everything was clear against Estonia in one evening," Koeman told Dutch television station NOS. "We still have six months. We have a lot of young players who will be half a year older. It will be a fight."
Koeman added that the March international window would see his side play two friendlies, which would heavily influence the squad make-up.
He also welcomed the gesture by captain-for-the-night and hat-trick hero Georginio Wijnaldum, who stood alongside teammate Frenkie de Jong in the celebration of his first goal, holding their arms out and pointing to the colour of their skin.
It follows allegations of racist abuse aimed at Dutch second-tier player Ahmad Mendes Moreira at the weekend during a game between his side Excelsior and FC Den Bosch.
Wijnaldum had spoken about the incident in the build-up to the Estonia match.
"Gini just said something about it in the dressing room during our closing words with each other," Koeman said.
"He said he feels so happy with this Dutch team, where this problem (of racism) does not occur. It is one team. It's nice to see that."