Simons, 23, ruptured an anterior cruciate ligament in his right knee during Tottenham Hotspur’s 1-0 victory at Wolverhampton Wanderers in the English Premier League last weekend.
“It is especially dramatic for the player himself, who is also fighting against relegation with his club,” Koeman told reporters while at an award ceremony on Thursday.
“The only plus in his case is that he is young. He will definitely play in major tournaments in the future, but that doesn't change the fact that this injury is dramatic.
“You could see from his reaction (on the field) that it could be serious ...
“We were waiting for the tests, and that turned out to be bad news. That is dramatic for us; he is a player with a lot of talent and potential.”
Koeman name-checked Bournemouth’s Justin Kluivert and Guus Til from Dutch champions PSV Eindhoven as potential replacements.
“It remains to be seen how Justin Kluivert's injury progresses. But in principle, we have enough options for the number 10 position.”
Kluivert had knee surgery after an injury in January, but told Dutch television last week he was back in training.
“An injury that is gone after a short period is not always bad news. The players have had a certain amount of rest. I don't think Frenkie de Jong's injury, who is now being eased back into the team at Barcelona, is bad news,” added the coach.
De Jong's absence from the European Championship in Germany two years ago was seen as a major blow.
“We are going to ensure the strongest possible side at the World Cup,” Koeman said.
The Dutch are drawn in Group F against Sweden, Tunisia and Japan - their opening opponents in Dallas on June 14.