Dortmund doesn't have a real CF beyond Haaland. He won't play when Haaland's not playing. If he does extremely well, he'll get some minutes as a direct back up to Haaland. If he doesn't play well, he likely won't play very much.
And he was going to be promoted to the Bundesliga squad for November when the rules allow for it, so the Champions League is no different. In fact, I'd say that it's easier to give him minutes in UCL. Realistically, Dortmund's not winning the UCL. We'll get past the group stage, and hopefully win a round or two from there. In the Bundesliga, we can't afford to drop points easily if we want a chance to beat Bayern. Harder to use a young player in that type of game, in my opinion.