The Red Devils underwhelming run continued on Thursday night as they were held to a goalless draw by West Ham.
It was the third time United have failed to score in their last five Premier League fixtures.
Mourinho opted to start Alexis Sanchez and Jesse Lingard up front against the Hammers, with Marcus Rashford and Anthony Martial benched.
Every time a Manchester United player misses a chance they always run back and look at Mourinho in the dugout – they look petrified, Merson told Sky Sports.
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. Its different at Manchester United – everyone is standing still, everyone just passes the ball – there is no movement. I think the players are petrified.
Mourinho is preparing for a huge summer as he looks to make the necessary changes to close the gap on rivals Manchester City.
Luke Shaw, Marouane Fellaini, Matteo Darmian, Juan Mata, Anthony Martial, Daley Blind, Marcus Rashford and Paul Pogba have all been linked with summer moves.
And Merson believes Mourinhos approach is pushing a number of players out of the exit door.
The former Arsenal and England star added: If a player gets sold in summer, they wouldnt leave Manchester United crying.
A lot of them look like they dont enjoy their football.
Manchester United finish their Premier League campaign with a home fixture against Watford on Sunday.
The Red Devils then turn their attentions to a FA Cup final showdown against Chelsea on May 19.