The Red Devils produced another below-par display against West Ham United on Thursday night as they failed to find a way through on goal in a 0-0 draw.
For the first time since March 2016 United were unable to score in back-to-back matches, having been defeated 1-0 by Brighton & Hove Albion six days beforehand.
Merson believes that a lack of confidence from United's attacking players is to blame, which he puts down to Mourinho.
"Every time a Manchester United player misses a chance they always run back and look at Mourinho in the dugout - they look petrified," he told Sky Sports News.
"You watch Manchester City, Tottenham, Chelsea and Liverpool play - and sometimes Arsenal - and you can see the patterns of their attacking play.
"It looks like there is a plan. It's different at Manchester United - everyone is standing still, everyone just passes the ball - there is no movement. I think the players are petrified.
"If a player gets sold in summer, they wouldn't leave Manchester United crying. A lot of them look like they don't enjoy their football."
Mourinho, who made eight changes to his starting lineup for the match at the London Stadium, must now prepare his side for Sunday's final league match of the season at home to Watford.