The German looked like a very astute signing for Arsenal, going 22 games unbeaten.
But since his return from injury in the new year, Mustafi has been shaky and loose in his defending.
He has a tendency to go to ground far too often when tackling, leaving himself and the defence vulnerable.
Arsenal fans blame conspiracy theory during Crystal Palace clash Mon, April 10, 2017
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Of the top six sides in the league, Arsenal have conceded the most goals and once again their defensive shortcomings were exposed.
The defence was all over the place for all three goals and it could have been worse with several schoolboy errors made in the 90 minutes.
Unless Arsene Wenger addresses the defence, Arsenal will always struggle.
Like in the last away game against West Brom, Arsenal were guilty having a lot of the ball but doing very little with it.
In truth, the lack of cutting edge against teams who sit deep has been a problem for Arsenal for years.
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Arsenal's Alexis Sanchez looks dejectedWelbeck and Walcott
The England duo had horrible nights for the Gunners, both barely having zero impact on the match.
Welbeck is meant to offer pace and a threat in behind but only once did he get beyond Palace's defence.
With his inconsistent goal record, he must much more.
Top Four gone
With this result Arsenal are all but certain not to qualify for the Champions League.
Even with the games in hand, there's no confidence in this Arsenal team to get the desired results.
A run-in which includes difficult games against Tottenham, Manchester United and Everton, Arsenal need a miracle to maintain Wenger's proud record.