Erik ten Hag's arrival at Manchester United last year began a process of culture reset. For years, the club allowed big egos to inflate, and the team's mentality to shrink, while an arrogant hierarchy seemingly assumed waving big cheques guaranteed success.
Ten Hag has taken steps to fix all of the above, and in the roughly eight months since he began working in May, the difference has been significant.
"There was no spirit," Ten Hag said last week. "I saw no team dynamic in the squad. The mental resilience was very low. I saw that as an outsider – and also noticed it in my first weeks at the club.
"I looked at the culture of the club. I asked, 'how did Manchester United become great?'. The club has bought an unimaginable number of players in recent years who have not been good enough. Most purchases have been average – and at United average is not good enough. United's shirt weighs heavily."
Of course, hindsight is 20/20, and you can't say Ten Hag's impact has come without "waving big cheques". But the problem with previous eras was how the money was spent.
Casemiro, who cost £60million, is the prime example. At 30 years old, there's no doubt some fans were unsure he was the man to reinvigorate a midfield that had quite literally been a problem for over a decade, but he's been exceptional and a big part of United's transformation.
From slow start to key man
Saturday's Manchester derby will be a true litmus test of not only United's progress under Ten Hag, but also the influence Casemiro has.
Let's not forget, City crushed United 6-3 at the start of October. Pep Guardiola's men were even 6-1 up for about 11 minutes before a late Anthony Martial double.
Their midfield of Scott McTominay, Christian Eriksen and Bruno Fernandes simply couldn't handle City's dynamism, and then Erling Haaland and Phil Foden were irresistible in front of goal.
That was, unsurprisingly, the last game before Casemiro took ownership of the holding midfield role at United. Casemiro has played 1,330 minutes across all competitions since, second only to Fernandes (1,349), while Scott McTominay has managed just 439.
Over the same period, only Newcastle United (24) have claimed more points than the Red Devils (23) in the Premier League, with November's 3-1 reverse at Aston Villa their sole defeat.
Of course, it's difficult to attribute United's improvement to Casemiro alone, but there's no doubt his effective blend of destructiveness and creative subtlety have made Ten Hag's midfield a completely different proposition.
Not only is he so adept at reading the game and snuffing out attacks, Casemiro's long-underrated technical abilities suit Ten Hag's style of play down to the ground.
More than meets the eye
Anyone who regularly watched Real Madrid during Casemiro's long stay will have already known there's more to him than simply kicking people. Admittedly, frequent viewers of arguably the most popular team on Earth is hardly a niche group, yet there was certainly a lack of awareness from fans and pundits alike regarding Casemiro's 'other' talents when he joined United.
Because Ten Hag wants his team to generally dictate possession, players without excellent technique will stick out like a sore thumb, which is presumably one explanation for Aaron Wan-Bissaka featuring so irregularly until the past couple of weeks.
The fact Casemiro has become so influential speaks volumes.
Every 90 minutes he averages 6.3 involvements in open-play passing sequences that end in a shot, a record bettered by only five central midfielders in the Premier League this term (min. 500 minutes), including more recognised creators like Fernandes (7.3) and Kevin De Bruyne (8.0).
Additionally, just five players in the entire league (min. 500 minutes) have been involved in more shot-ending build-up sequences (48) without creating or taking the shot. Both of these highlight how central Casemiro's playmaking skills are from his deeper role, even if he's not necessarily the one playing the key pass.
But he is proving extremely effective without the ball as well, and his powers of ball recovery combined with smart distribution make him such an asset, with only Rodri (32, from 1,391 minutes played) initiating more shot-ending open-play sequences after winning possession than Casemiro (22, from 979 minutes played).
It's arguably that hard-working, destructive nature that makes him so refreshing for United, though. The only other central midfielder they've had over the past 15 years who has recorded at least three tackles and eight duel wins (3.8 and 8.1) on a 90-minute basis over a season is Marouane Fellaini in 2013-14 (3.1 and 9.3) and 2016-17 (3.0 and 10.8).
Fellaini's stats will be boosted by his aerial effectiveness, and obviously the Belgian never had the same technical grasp Casemiro has, with his two brilliant passes in the build up to Marcus Rashford's goals in the EFL Cup win over Charlton Athletic earlier this week prime examples of his class in that respect.
He's probably the most complete midfielder they've had since Roy Keane, and the fact Ten Hag so emphatically filled a void that's been gaping throughout the post-Alex Ferguson era is proof enough of the manager's culture shift at Old Trafford.
Beating City will be another major statement.