Problem you immediately have is that your defence is worryingly weak. The only talent in that area, by your reckoning, is Bellerin, and Xhaka is only accommodated with reservation. The question is whether he can get those attacking players to press effectively for 90 minutes without leaving the weaknesses at the back exposed and without burning the attackers out. Lots of good individual players, on their day, but little evidence in the last two seasons to say that they are willing to work for each other as a unit.
Yep mate, it's fairly obvious defence is our weak point. Think we had the joint 3rd best attack in the league and that's wuth Aubameyang only being there a few months, but the defending was really poor at times.
To be honest, it's hard to judge individuals because of our lack of tactics. Bellerin was promising enough that you guys and Barca were linked but has stagnated, Mustafi was a German international but has went off the boil totally, Koscielny was a starting defender for France, Kolosniac was in the Bundesliga team of the year, Monreal is in the Spain squad. Then we have youngsters like Chambers, Holding & Mavropanos.
Xhaka himself excelled in Germany and for his country with a more defensive midfielder alongside him, but we expose him as the defensive midfielder and play him in a 2 with the most forward thinking centre in the country.
The lack of tactics, and the lack of balance meant that we were all too often exposed and looking amateur.
So I agree, his main challenge for him is to get the balance corrected in the team, and to get players who defended as individuals, and when they could be bothered it seemed, into defending as a team.