The Brazilian is set to miss several weeks due to a hamstring injury during a phase when Erik ten Hag needs him most. United bought in Sofyan Amrabat for such a scenario and a good spell may see him become first pick.
The sharpness of players naturally degrades with age but the rate of Casemiros drop-off from last season is unexpected and alarming. The midfielder turns 32 in February so time is not on his side.
United shelled out an initial 60m for the five-time Champions League winner in a transfer that could see the fee increase by 70m. Upon his arrival, Roy Keane was well aware of the repercussions of this.
I think it is a good signing, they have paid a lot of money and he is 30 years of age with a five-year contract, he said, before adding: I think hes a quality player, you could look at the bigger picture but obviously its a lot of money, a long contract.
The agents out there must be delighted theyve got Man Utd. But when youre desperate, youve got to pay over the odds. United were indeed desperate and Casemiro was arguably the reason they achieved a top four spot last season.
However, he has missed a total of 11 games so far, eight of which were for suspensions last season. That figure is set to increase and with it, Uniteds investment will continue to depreciate.
United need Casemiro to be on his game for more than one season to make that 70m fee worth it. Or else, as Gary Neville pointed out last year, United could have landed themselves in some bother.
[Hes] on a phenomenal contract thats going to cost the club 160m, the Sky Sports pundit said of Casemiro. I think hell do well for the club for a couple of years.
But someone, in two years time, is going to pick up a player thats beyond his best years, on 20m a year that they cant shift. That might be Erik ten Hag, a new manager or new owners.
There is a large element of desperation with the signing because a five-year contract for a 30-year-old should not happen.
United have a knack of losing enormous amounts of money on transfers. Record arrival Paul Pogba was signed for 89m and left for nothing. Around 58m was spent on Anthony Martial and he could leave for free next summer.
So what does the future hold for Casemiro? It is unimaginable to think he will be a key starter by the age of 35 and so at best he will be a rotation player. More realistically, he may be surplus to requirements but there is the possibility he stands firm and waits out his contract.
But even in the event he does move on, United will only reap a portion of what they shelled out due to his age. And that is without possible persisting injuries.