I agree that his overall package as an Arsenal CB is not good enough. But every criticism about his defensive work in the last two games are applicable to the whole back line.
When will we learn that defending is about collective shape and organisation, which includes the midfielders and forwards, not just individual central defenders?
Mustafi is a loss but for me Gabriel has performed no worse (purely in terms of defensive duties) than Koscielny has this past week.
Our teams defensive failures are down to a lot more than him, and we were still failing to hold clean sheets with Mustafi in the team.