Cavani, who scored 200 goals in 301 appearances for French champions PSG, is set to sign a two-year deal at Old Trafford.
Manchester United have been persistently linked with a move for Jadon Sancho this summer but the club have turned their attentions elsewhere, with Borussia Dortmund insisting he will not be sold.
Cavani has won a number of trophies during his career but Merson has criticised Manchester United’s transfer move and claimed he could disrupt Greenwood’s development at Old Trafford.
Greenwood enjoyed a breakthrough season during the 2019-20 campaign, scoring 18 goals to help Manchester United finish third in the Premier League.
Asked about Cavani’s most to United, former Arsenal and England star Merson told Sky Sports: ‘For me it’s a panic [buy].
‘I hope I’m wrong but it’s not a future signing and it’s not one that tells me they’re moving forward.
‘It’s not one that tells me that in three or four years they’re going to catch up with Liverpool and Man City. This is not going to catch those two clubs up now.’
Merson added: ‘It’s definitely a quick fix – he’s 33 years old. You’ve got Mason Greenwood already there who’s going to be a proper player so is he going to hold him back?
‘Cavani was a great player but he’s 33 years old. How many players are playing in the Premier League now at that age?
‘You’ve got to have pace as Man United like to counter-attack with the pace of Anthony Martial, Greenwood, Marcus Rashford and Dan James when he’s in the team.
‘Now, all of a sudden Cavani comes in. He’ll hold the ball up, but for me it’s a panic.’
Manchester United suffered an embarrassing 6-1 defeat to Jose Mourinho’s Tottenham on Sunday.