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Ahead of Saturday’s match at Wembley the Gunners are in the midst of a defensive selection crisis.
Laurent Koscielny is suspended, Gabriel is out injured and Shkodran Mustafi is struggling with concussion.
It means the inexperienced Rob Holding and Mertesacker, who came off the bench last weekend to make his first appearance of the season, are expected to fill the centre-back berths.
Arsenal legend Merson is very worried.
‘If both teams had brought their A game then Chelsea would still have won, but with Laurent Koscielny and Gabriel out, Shkodran Mustafi potentially out and Per Mertesacker playing at the back I would be shocked if Arsenal got a result,’ he told Sky Sports.
‘Arsenal need to have their best defenders playing to have any chance and they haven’t.’
It is the returning Mertesacker, who has been out injured virtually all season, who Merson sees as key.
‘Mertesacker is going to be crucial for Arsenal if he plays; he will have to play very well if Arsenal are to have any chance.
‘If he doesn’t play well then Chelsea are going to cut through Arsenal like a knife through butter. The Arsenal defenders are going to have to have their best games of the season. If they can do that then you never know.’