The Red Devils have continued to struggle after the German boss left the club at the end of the 2021-22 season having guided the side to a record low Premier League points total of 58.
Last campaign they finished in eighth spot on just 60 points with Ten Hag only clinging on to his job thanks to a victory over bitter rivals Man City in the FA Cup final.
And this season is not looking much better, with United already having slipped to two bitter defeats, including a humiliating 3-0 loss at home to Liverpool, in the first three matches.
Now former manager Rangnick has arrived to rub salt in the wounds, doubling down on the 'open heart surgery' remarks he made on his departure, in an interview with TV2.
He said: 'I probably made that statement when the season was almost over about two years ago.
'It is probably only two or three weeks since Erik ten Hag was asked the same thing.
'Then he said that I was right in my analysis. I don't know how many players they have signed since, but it looks like I was right.'
The comments to which he was referring were made in April 2022 after United slipped to a 4-0 defeat at Anfield in one of Rangnick's final matches at the helm.
Following that match he said: 'We need positive energy for the new team and the new manager.
'It's not the manager's quality - it's changing all the other things that have brought the club into a position we are in right now.
'The good thing - one of the few - is it's crystal clear. You don't even need glasses to see the problems.
'Now it's only about how you can solve them. Not minor cosmetic things. This is an open heart operation. If everyone realises this has to happen and works together, it doesn't need to take years.
'It can happen within one year. Other clubs have shown it's possible within three transfer windows.'
United face a trip to Southampton in their next match on Saturday, as Ten Hag looks to kickstart the side's season
Rangnick was in charge of United for 29 matches as caretaker boss between 2021 and 2022 following the dismissal of Ole Gunnar Solskjaer.
He was set to move into a consultancy role at Old Trafford at the end of that season but it was decided he would not have the time to fulfill these duties given his appointment as the manager of the Austrian national team.
United face a trip to Southampton in their next match on Saturday, as Ten Hag looks to kickstart the side's season.