Why I'm not so sure about Dortmund is not only because of their performance in the Bundesliga. But you can compare the opponents that Dortmund faced in the Champions League in the qualifying round were not strong enough and favored teams. Maybe only Madrid and Barcelona were their opponents from strong teams. And Dortmund lost to both of them.
Facing Sporting I think Dortmund could also have problems. Sporting is not a strong team if you look at their performance in the Champions League, both of them might show a tight game. It will definitely be an interesting match.
I don't think so. While Borussia Dortmund has had a series of mixed performances this season, it doesn’t change the fact that they are still a stronger side than Sporting CP in the Champions League. Sporting have equally struggled especially since the departure of their Ruben Amorim to Manchester United. Although the match will be tightly contested as Sporting will have home advantage in the first leg, Dortmund still has the quality to edge them out over two legs provided they find some consistency in their performance.