United were thrashed 4-0 by Liverpool in midweek, a defeat which left them in sixth place in the table, 23 points behind leaders and neighbours Manchester City.
They also face an uphill struggle to reach the Champions League next season.
“One of the very few good things, is that it’s crystal clear. You don’t even need glasses to analyse and to see where the problems are,” said 63-year-old Rangnick.
“So, now it’s only about how do we solve them? For me, it’s clear it’s not enough to do some little, minor amendments, some little issues here and there, some minor cosmetic things.
“No, in medicine you would see this is an operation at the open heart, so there are more things to be changed than some little things here and some minor things there, and this is the good thing.”
Ajax manager Ten Hag will face a huge task next season to revive United after a fifth consecutive trophy-less campaign.
The 52-year-old Dutchman becomes the fifth permanent manager at Old Trafford since Alex Ferguson’s retirement in 2013 sent the 20-time English champions into decline.
Ten Hag is on course to win his third league title with Ajax and took the Dutch club to the semi-finals of the Champions League in 2019 for the first time in 22 years.