Wenger is really to blame for the loss. You can't press high up the pitch with a slow central defender unless yours is a far better team than your opponent's. You are just begging for that central defender to be left isolated during a quick turnover of possession.
You can see Chelsea have trained for this vulnerability for years. It is obvious you need two quick central defenders to play Chelsea.
Whereas when Arsenal play Man City they adopt a deep defensive set-up. In those circumstances, Mertesacker is fine.
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