The Greece international was hurt during last weekend’s goalless draw at Union Berlin, having entered the match as a second-half substitute. The injury was initially a concern, but Dortmund later confirmed it was a ligament problem in his foot and less serious than first feared.
Kovac said the club would not rush the 18-year-old back for the next match either, even if he makes progress during recovery.
“We will not have him with us tomorrow. We will have to see about Bremen; I cannot say anything about that yet, because we will not push anyone into a situation that could be too dangerous.
“It is better to rest for one more game than perhaps then be out for a month.”
Dortmund face Werder Bremen in the DFB-Pokal on Tuesday, giving Karetsas another possible return date. But the coach made clear that the decision will depend on how the player responds rather than the importance of the fixture.
The club have reason to be careful with a young summer arrival who has already endured an interrupted start to life in Dortmund. Karetsas was taken off shortly after coming on against Union, and club officials have stressed that they will assess him day by day.
Before the injury, the midfielder had shown flashes of the attacking energy that persuaded Dortmund to bring him in from Genk. He made an impact after coming on in Berlin, winning duels and trying to drive the visitors forward before his evening ended prematurely.
Dortmund’s immediate priority is to ensure that the ankle problem does not become a longer absence. Missing Frankfurt is now confirmed, while a place in the squad for the cup tie remains uncertain.
Kovac’s comments suggest the club will favour a conservative recovery plan rather than gamble on an early return. For Karetsas, the next step is to regain full fitness before he is considered for selection again.