According to a recent report by Sky Sports, the Valencia defender has agreed to join the Gunners this summer though an agreement on a fee between the clubs has yet to be reached.
Arsenal have been in the market for a new defender for much of the summer as boss Arsene Wenger looks for a reliable player to partner number one centre-back Laurent Koscielny.
Merson, however, questions why a deal for Mustafi wasn’t struck as soon as the European Championships had finished and suggests he could be seen as a last resort signing on the eve of the new season.
"Why didn't they buy him when the Euros had finished, if they fancied him that much? It feels as if it is a bit of a last resort,” Merson told Sky Sports.
Arsenal have had all summer to bring in the necessary players for the coming season, yet heading in to the Liverpool match this weekend many feel they are still short both in attack and defence.
Mustafi, a German international, had a previous spell at fellow Premier League side Everton but after struggling for first-team opportunities he moved on to Sampdoria and later Valencia.
Last season he made over 40 appearances in all competitions for Los Che, and his decent form earned him a place at Euro 2016 where he managed to find the net in the Germans’ 2-0 group match win over Ukraine.