The Red Devils brought in the Uruguayan on a free transfer on deadline day as he put pen to paper on a two-year deal at Old Trafford.
Cavani has been without a club since his contract at Paris Saint-Germain expired in June, and at 33 years old, the striker brings a wealth of experience to his new post.
The Uruguay international left PSG as their all-time top scorer, netting a remarkable 200 goals in 301 appearances.
However, United’s major problems this season have been in defence, which was humiliatingly showcased by their 6-1 defeat at home to Tottenham last weekend.
Merson says Cavani won’t make the difference and without the adequate defensive reinforcements, the Red Devils are currently not a top-four club.
‘He’s been available on a free transfer since I don’t know when, and I don’t think they got beaten by Tottenham because they can’t put the ball in the net.
‘Having a man sent off in that game just gave them an excuse. They were already all over the shop defensively.
‘I thought they were a shoe-in for the top four before the season started but until they get some legs in that back four I don’t see where they are going.
‘They have opened the door for teams like Arsenal, Tottenham and Everton to finish in the top four this season.
‘I don’t think they are a top-four club at the moment. They can’t defend for toffee, and they let Chris Smalling go to Roma. He would probably get in the team right now.
‘Cavani feels like a Quick Fix. But he’s probably not even that if I’m being honest. I don’t see him being the difference.’