But after sitting in the stands at Old Trafford to witness Monday's 2-1 victory over arch-rivals Liverpool, the Brazil international probably wondered what all the fuss was about.
For 90 minutes United looked like a top football team again -- roared on by a passionate capacity crowd, they outran, outfought and at times outplayed a Liverpool side considered to be genuine title contenders.
The protests against the club's owners, the Glazer family, before and during the game, were a reminder that there remain many deep-seated problems at United which a one-off victory, certainly doesn't mask.
But for Dutch manager Erik ten Hag, the win was a massively needed boost after the embarrassing start to the season which had seen a home defeat to Brighton and Hove Albion and a crushing 4-0 surrender at Brentford.
And for Casemiro, there was evidence that he will have team mates who do, after all, have the capacity to compete and win against the Premier League's elite.
Casemiro, a tank-like destroyer at the base of Real Madrid's midfield in five Champions League winning campaigns, was signed to bring some badly needed aggression, nous and resilience to the centre of the field.
But what was most impressive about United's display against a badly below-par Liverpool was the addition of steel provided by Ten Hag's first two signings.
Central defender Lisandro Martinez has already faced doubts and criticism after his performances in the opening two games but right from the outset, when the Argentine left his mark on Mohamed Salah, he was a determined presence.
Playing alongside French international Raphael Varane, who replaced the out of form Harry Maguire, Martinez contributed to a solid display that showed none of the hesitancy and uncertainty of so many United performances at the back in the past year.
Dutch left-back Tyrell Malacia, making his first start since joining from Feyenoord, got over his early nerves to quickly win the appreciation of the home crowd with his tenacious tackling and tireless running.
With Casemiro set to come into the side at Southampton on Saturday, United should be a tougher and better organised team defensively and Ten Hag will hope that injects some fresh confidence into the attacking half of the side.
In attack, with Cristiano Ronaldo benched, there were encouraging displays from Marcus Rashford and Jaden Sancho -- both scored to inject some confidence into two players who have looked short of self-belief.
"I'm convinced they can be really a threat, really a weapon against every opponent. As long as their focus and energy is there with them, it is very difficult to stop them," Ten Hag said on Tuesday.
"So it's about the team getting them in the right situation and also about themselves, with the right approach, right focus in the game and battling, defending, pressing. But also dribbles and combinations and runs in behind," he added.
But what pleased the Dutchman the most was seeing a United side, written off as no-hopers last week, find the fight and aggression for battle.
Now he wants to see that approach delivered consistently.
"What I demand is that players bring the top spirit every day. Every day, they have to bring it to the club," he said.
"It has to be a way of life and I hope they understand. It's only working - high performance, high level - if you have that spirit and have that focus."
For 90 minutes United had all of that. But fans who have grown weary of false dawns, will want to see evidence that Ten Hag's message truly had got through to the players.