The Red Devils were one of the only top teams not to bring in any reinforcements during the mid-season window despite being linked with a host of star players during the course of the month.
Louis van Gaal has found himself under intense pressure at Old Trafford in recent months following United’s stuttering form in the Premier League.
The Red Devils are currently five points adrift of the top four and have only won two of their last 10 top-flight games.
Former Arsenal man Merson was surprised that United opted not to make any signings last month.
Asked to rate United’s January transfer business, Merson told Sky Sports: “I’m very surprised they haven’t signed anybody and now with Pep Guardiola going to City, and Liverpool and Man City reaching the Capital One Cup final, it doesn’t get any better for them.
“You have to give them a D because they haven’t done anything.
“If a kid doesn’t do his homework then he’s going to get a D – and they haven’t done their homework.”
United are preparing to welcome Southampton to Old Trafford in the Premier League on Tuesday night as they look to get back to winning ways in the top flight.
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