Of course, no coach will run away from the challenges ahead of him, he's supposed to learn more from the game. Coming up with a tactics to play with, and also understand the ongoing game. That will give him the right opportunity to substitute players and cover up whatever lapses on the field. The problem is that Madrid pressure will be too much on Arsenal defense.
In the Champions League there will always be pressure for any team so that it is a very real challenge for coaches and also for their players so it is very much expected for every coach to be able to provide more appropriate tactics when facing certain teams in the Champions League. I also assume that Arsenal will get fiercer resistance from Real Madrid because the mentality of Real Madrid players has indeed been formed to perform well in the Champions League by following the instructions of their own coach.