That was a reckless tackle by Casemeiro and his decision cost the team to lose, some may think that he did that for the team to prevent the goal but to me its a poor decision because a goal scored can always be equalized. The team suffered and they were in even more pressure and the story could have been different if the penalty went in.
I don’t think with Manchester United’s performance currently they can survive the group stage, they need to improve their game if they really want to qualify to the knockout round. They are not in good shape both in the Premier league and other competitions but no one can tell where the problem lies but I think the coach is not doing enough.
Don't blame Casemiro for the action that led to the penalty. It was Onana's fault because of the careless clearance of the ball which had to enter the legs of Mertens and Casemiro had to see if he could do something to save the team and as Mertens knew he was coming, ran into his front and that penalty was awarded. As it is, Onana is not living up to expectations and I can see frustration deeply liming his features. All he needs right now is to be relaxed, some games he should not start, so he can be able to ease off steam because at this rate of over use he'd not be able to pick up form.