Signed from Real Madrid, Casemiro made his first few Premier League appearances from the bench as United experienced some wildly varying results.
Ten Hag was patient with the marquee signing, bedding him in slowly, and naming him only as a substitute in United's 6-3 Manchester derby defeat at the Etihad in early October.
But, since his first league start at Everton the following weekend, the Brazil international has quickly become one of Ten Hag's most important players as United's form sparks tentative talk of a title challenge.
Casemiro has started each of United's last 11 Premier League games and played almost every minute as the club have shot up the table, enjoying their best run of form since Sir Alex Ferguson was in charge.
"They would have been a fragile team in that environment there, you could see there were cracks but you can see there's a togetherness and it's all knitted together by Casemiro," Ferdinand told BT Sport after United's comeback victory in the Manchester derby.
"I think he's been the most important part in the whole jigsaw for them. It's by no means finished, but he's been a really important part and the glue that sticks everything together. It all goes through him."
United have arguably been crying out for a world-class defensive midfielder since the days of Sir Alex Ferguson and the 30-year-old Casemiro has certainly elevated his side's performance since arriving at Old Trafford.
The midfielder has slotted into United's seamlessly and appears to enjoy working under Ten Hag.
"He's certainly a manager who is obsessed with winning," Casemiro said of Ten Hag ahead of United's 2-1 comeback victory against Pep Guardiola's Manchester City on Saturday.
"Few managers in my career had the same obsession to win. He's someone who always wants to win, even in training, he always wants the best.
"He's a hard-working manager who demands a lot of his players, so that we're always at our best and that's important for us. I also view him as a manager with the desire to see Manchester United grow and develop – that's one of his most important characteristics.
"Not just his, but all the coaching staff who follow his lead. He's a new manager here at the club, he's trying to introduce his philosophy and style. It's still very early days in terms of football, still early in terms of introducing his philosophy and bringing in his own players.
"We are growing, we are developing very well and understand the manager better, this is really important to win trophies. We know that it's a process, it's very important to adapt to this process quickly."
To illustrate how influential Casemiro has been, we've compiled the Premier League table since his first start in the competition. United might finally be back.
1. Arsenal – 10 games, 26 points, +16 GD
2. Manchester United – 11 games, 26 points, +11 GD
3. Newcastle United – 10 games, 24 points, +14 GD
4. Fulham – 12 games, 20 points, +5 GD
5. Brentford – 10 games, 19 points, +5 GD
6. Liverpool – 11 games, 18 points, 0 GD
7. Aston Villa – 11 games, 17 points, -1 GD
8. Brighton – 10 games, 16 points, +5 GD
9. Manchester City – 9 games, 16 points, +4 GD
10. Nottingham Forest – 11 games, 16 points, -4 GD
11. Crystal Palace – 11 games, 16 points, -6 GD
12. Leicester – 10 games, 13 points, +2 GD
13. Tottenham – 10 games, 13 points, 0 GD
14. Chelsea – 11 games, 12 points, -2 GD
15. Wolves – 10 games, 11 points, -6 GD
16. West Ham United – 11 games, 8 points, -6 GD
17. Leeds United – 11 games, 8 points, -7 GD
18. Southampton – 10 games, 8 points, -8 GD
19. Everton – 11 games, 5 points, -11 GD
20. Bournemouth – 10 games, 4 points, -11 GD