The Greece international, one of the club's major signings this season, suffered an ankle injury on October 17th in the 0-0 draw with Union Berlin. He had just returned after collapsing in September due to what the club called 'circulatory problems'.
"He trained in part with the team today. It was matchday plus two and minus two so it was not too intensive," Kovac told a press conference.
Dortmund moved into the German Cup third round with Tuesday's last-gasp 3-2 home win over Werder Bremen.
"Today was the first time Karetsas trained with the team. The plan is that if all is well tomorrow, he will train with the team and we will decide if he is ready to go to Munich or not," Kovac said.
Dortmund, like Bayern, are still unbeaten in the Bundesliga this season, and both teams are on 17 points, with the Bavarians top on goal difference. Dortmund have scored 12 goals in seven league games, fewer than half of Bayern's 25.
"It is the Klassiker, and after a long time we can play a game having the same level of points," Kovac said.
"That makes the match more interesting, but it does not make it easier. Bayern are the gold standard, not only in Germany but in Europe.
"We may have a chance this time to turn the game in our favour but, as I said, it won't be easy."