"A season like Ajax have just had is priceless for the development of players," he said of the club's exploits in the Champions League, where they let a place in the final slip in the last minute of their semi-final tie against Tottenham Hotspur, plus their domestic league and cup double.
"Donny van de Beek is the best example of that. He has come on such that he is knocking on the door of selection, much louder than before."
Van de Beek is one of four Ajax players in the Dutch squad for the Nations League, where the Dutch meet England in the semi-final in Guimaraes on June 6.
In the last outing for the Dutch against Germany in March, Koeman picked a midfield with Frenkie de Jong, Georginio Wijnaldum and Marten de Roon, while the 22-year-old Van de Beek was an unused substitute.
Koeman said he was pleased with the Nations League preparations despite it coming at the end of a long and arduous season for many.
"If I see how they've trained, then I can say no-one has held anything back. The spirit in the squad has really grown and players really want to be a part of that. That's been good to see," he told a news conference.
"The Nations League is not the most important prize that there is, but it is a trophy nevertheless that we'd like to win. Plus we've got a chance to play two more good matches."
The winner of the Netherlands versus England faces the winner of the June 5 clash between hosts Portugal and Switzerland on June 9 in Porto. The losers of the semi-finals will clash for the bronze medals in Guimaraes the same day.
(Reporting by Mark Gleeson; Editing by Hugh Lawson)
