The Gunners ended up bringing in midfielder Mohamed Elneny from FC Basel as their only signing during the mid-season window.
Arsenal are currently third in the Premier League table and three points behind leaders Leicester City heading into their clash with Southampton at Emirates Stadium on Tuesday night.
But former Arsenal man Merson admits that he is worried about the north London club’s squad depth as he ran the rule over the club’s transfer business.
Asked to rate Arsenal’s January transfer business, Merson told Sky Sports: “They get a C because they didn’t do enough. Mohamed Elneny is nowhere near enough.
“They definitely needed a defensive midfield player, but also a forward. If Olivier Giroud gets injured then can Theo Walcott come in and score enough goals in the last 15 games?”
Arsenal head into Tuesday’s clash with Southampton on the back of their FA Cup victory over Burnley at the weekend.
The Gunners will take on Hull City in the fifth round of the competition later this month as they look to lift the trophy for the third season running.
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