Schlotterbeck underwent surgery five months ago for a meniscus tear in his left knee that he sustained in April, and despite having come back to light team training in recent days, he is still not yet ready for a comeback.
Schlotterbeck's services were also badly missed in Germany's two World Cup qualifiers in the international break, with the national team suffering a shock 2-0 loss to Slovakia before beating Northern Ireland 3-1.
"Nico has come back after five months and has been doing some light training without contact," Kovac told a press conference.
"He is a key player, a top national team player.
"Before we even consider the national team, he has to be healthy for the club and have playing minutes in training, in Bundesliga and Champions League matches," he said.
"It will take more time. A Bundesliga player and especially a defender has to be able to do one-on-ones if they want to be part of the bench, and he is not yet that far," Kovac concluded.
Dortmund, who had a shorter pre-season due to the Club World Cup, travel to Heidenheim in the Bundesliga on Saturday, having four points from their two league matches so far.
They also kick off their Champions League campaign on September 16th at Italy's Juventus.