Dortmund might have a poor performance in the Bundles league but it turns out that they are better in the champions league competition. There are similar teams with same scenarios like this and that is why I won't underestimate Dortmund in the champions league competition.
If the head 2 head count for both team reads Dortmund winning three out of 4 matches which both played against each other then we should expect a very fierce game that could either lead to extra time or a direct 90' win because Dortmund still have strong advantage over sporting despite their poor bundles league performance.
I agree with you about Dortmund chances in Champions League. Even though they are not doing well in Bundesliga they seem to play better in European games. They have beaten Sporting three out of four times they played each other which means they might feel more confident going into game. This, plus their experience in Champions League makes them tough team to beat. I think it will be exciting match that could go either way with Dortmund momentum possibly helping them win either in regular 90 minutes or in extra time.