Casemiro's game often endangers his own club, it is very unfortunate that the red card really put Manchester United in danger, this defeat was really surprised them and put United in the bottom standings so it will start to raise doubts that they can survive in the Champions League. because losing in a home match is a big loss.
Casemiro has no choice after Onana passed the ball to him without accuracy and he only tried to tackle to block that chance of Galatasaray. Without that tackle, very likely Galatasaray will have a goal but with a quick situation, I think we can not ask Casemiro to be calm and don't make that tackle.
The biggest responsibility belongs to Onana when he had many options in that situation but chose a hardest choice. Only Onana knows and can explain why he decided to make that pass.
What Casemiro made was due to Onana's own mistake for passing to the opponent while Casemiro behind him tried to defend by making a tackle, as a result it became a penalty even though the Galatasaray player failed to execute it.
What is done is done and can not be changed but after some bad matches so far, Onana currently is facing with huge pressure. I wish him the best and hopefully he won't be a failed case like Maguire. He still has time to prove his ability and contribute for the club but must play more careful with his feet and sharpen his saving skills more.