Dortmund scores an average of 2.5 goals per game in the last 10 games, so we can assume that with such an opponent as Schalke, the figure will be at least two goals. But I don't think that now anyone cares about goal difference and secondary statistics, especially when there are important games in the Champions League and there is still a long distance to the end of the championship that will require a lot of strength.
That's right, this week is a busy schedule for Dortmund and I think they can win with an average goal alone which is good and Dortmund doesn't need to play hard to score many goals. Coach Eden Terzic must be able to work on the stamina of his players so that everything is kept safe and stable in every match that will be played later. Player rotation and long-term implementation must be maintained by Dortmund in order to get maximum results.
By the way, I remembered my idea that Dortmund will play either in a great mood after reaching the 1/4 of the Champions League, or in a terrible mood after relegation. In both options, they should play well. Despite the fact that Schalke in recent games almost does not concede goals, theoretically they should return to the average result in the season - almost 2 goals per game. Perhaps I will bet on the victory of Dortmund with a handicap of -1 and odds of 2.46.
I think it comes definitely down to whether or not Dortmund scores a fast first goal. Now that Schalke has only conceded one goal in six games, it doesn't seem to me as if it is still coincidence and not a fundamental change in their defense as you say. I think they are just better, which means that they are not necessarily going back to their average of conceding two goals per game. It can't just be luck if have five clean sheets in six games when before you conceded two per game. I did maths and it was even 2.5 goals conceded per game on average.
I am sure they must have done work to improve it. You can be lucky once or twice, maybe three times in a row, but is conceding one goal in six games still luck?
I tend towards Dortmund as well as any other result would be a disappointment for them. But there are now a couple of teams who thought it's going to be some easy three points against Schalke and then they couldn't score.