Ajax opened the new season of the Dutch Eredivisie on Saturday night without Ghana star Mohammed Kudus.
The return of Kudus is taking longer as Ajax were hoping he would be fit for their opening league match of the season.
The Ghanaian suffered an ankle injury in pre-season preparation which has forced him out of action.
"That is incredibly unfortunate," said Ten Hag, the coach of th Dutch giants.
"He was doing very well. I don't know when he will be back. I always think he can join training again, but then it is disappointing again."
Kudus has been called into the Ghana's national team for next month's 2022 World Cup qualifier.