I don't if he would be a good defender for Manchester United without injuries, which has mostly affected his game since leaving Ajax for Juventus. I know this move for this player has to do with Erik Ten Hag, this is one of the defenders that partnered with Lisandro Martinez season after season.
Yeah there is that and besides de Ligt is a Dutch player as well so I also believe Erik ten Hag has intentions to bring him to Manchester United.
I believe Lisandro Martinez and de Ligt would make a really great duo in the defense line. They both worked with Erik ten Hag at Ajax. Defense is the immediate area to improve for the Red Devils so this would be like a medicine for them.
Source: https://www.manutd.com/en/news/detail/raphael-varane-announces-departure-from-man-utd
Varane has announced his departure as you must have heard. He hasn't been playing like in the past anymore. He has been quite inconsistent so I believe this is the best for both sides. Filling his place with a defender like de Ligt would be perfect!