The five-time Champions League winner will swap LaLiga for the Premier League and sign a four-year contract in a deal worth £60million.
The Brazilian is seen as a huge coup for United after playing a vital role in Madrid's European dominance over the past decade, and will likely slot in as one of the first names on the team sheet.
Erik ten Hag's side have got off to a dreadful start to the season with losses against Brighton and Brentford, and former Premier League striker Tony Cascarino believes the 30-year-old Brazilian could be in for a shock when he realises the quality, or lack of it, around him.
The talkSPORT host said: "A lot rests on his shoulders in terms of making a big improvement to the team.
"Casemiro can't really run, he's not a running midfielder so he needs legs around him for a start. If you're going to buy Casemiro make sure you've got loads of energy and legs around him. The one thing you've heard about United the last few weeks is that they don't run."
Cascarino then drew an intriguing comparison to another midfield superstar who joined in 2015 and quickly realised the task at hand.
Bayern Munich allowed club legend Schweinsteiger to leave the Bundesliga for Old Trafford with the German boasting a similarly impressive CV to Casemiro.
But the World Cup winner, who also moved at the age of 30, struggled from the off and left after after just a season-and-a-half with only 35 Red Devils appearances to his name.
Cascarino believes the comparison is there, due to the lack of players willing to run in United's team.
"It reminds me a bit of Schweinsteiger," he said. "When Schweinsteiger was at Bayern he was a terrific player.
"He came to Man United and his legs weren't quite the same, maybe that's harsh on Casemiro but Schweinsteiger needed legs in the team for him to shine and he never got going.
"I think [Casemiro] will come to Man United, he'll see what's happening and quickly realise it's a million miles from Real Madrid, the players, the quality of the play.
"I don't think he's a player that's suddenly going to hit the ground and make a huge amount of difference.
"The problem is they've got a lot of players who don't want to run, his job is going to get even harder, you might see him get bypassed. Schweinsteiger got bypassed a lot at Man United and that team ran a lot more than this one does."