The Red Devils confirmed on Friday evening that they have reached an agreement with Real Madrid to bring the Brazilian midfielder to Old Trafford, with the attached fee understood to be somewhere in the region of £70 million.
But amid a period of mounting frustration for United both on and off the pitch, the Sky Sports pundit has questioned why the 30-year-old has opted to leave last season’s Champions League winners to move to Erik ten Hag’s struggling side.
When asked where United needed to focus their attentions as they attempt to rebuild under Ten Hag, Merson told Sky Sports’ Soccer Saturday: “It’s a difficult one. It is a difficult one. At the end of the day, the players are the ones who win you games. I don’t know where they start.
“They’ve started with Casemiro. He’s a winner, so you start with a winner.
“But he’s 30 years of age. I feel with this transfer it was like, ‘Man United have come in for me? Alright, just ask for silly amounts money, and if they say yeah, I’ll come, and if they don’t, it’s no loss’. I just don’t get it.
“He’s just won the Champions League, and now he’s not going to be playing in the Champions League. Four-year contract - he probably wouldn’t have got a four-year contract at Real Madrid.
“There’s so much work to be done, so so so so much work.”
Merson also assessed United’s chances heading into Monday’s meeting with bitter rivals Liverpool, suggesting that the Red Devils’ current slump in form and the low standards they have set already this season makes the clash something of a “free hit”.
Addressing the upcoming match, he said: “The big advantage they have on Monday? All they’ve got to do is run around. That’s all they’ve got to do.
“They haven’t got to beat four players, or do this or that - just give everything they’ve got, that’s all they’ve got to do.”
Neither United or Liverpool have won a game so far this season, with Jurgen Klopp’s men drawing their first two outings of the new campaign.