It's absolutely normal for professional players to have days where they underperform and if you ask me, I think Casemiro did have one of those bad days at the office last weekend when Manchester United played against Liverpool. The Brazilian have faced intense criticisms since then by many especially Manchester United fans and there was even a report that he was linked with a move to Galatasaray after that game.
You are right that there are time when some professional players perform bad because I have seen so many professional players underperforming sometimes, so actually in football no matter how good a player is there would always be a time when there performance will depreciate, even Messi there are some matches he perform lower than he normally do but the good thing about it is that there performance will always remain intact.
However Casemiro is a very important player but I don't no why I always feel that Manchester United was not meant for Casemiro because a lot of things has changed about him because he has become more weaker than when he was on real Madrid and even the popularity he is no longer that popular, though he is trying for Manchester United but he is not getting the kind of assistance he needs, perhaps joining other club like Manchester City would have been more better for him.