The way I interpreted it (wrong or not) was that the keeper made the save and error, then attempted to play the ball and it was poked away during the effort to retrieve it, causing contact with only Morata and not both.

The only way it’s red is if the referee believes the keeper deliberately plays Morata instead of the ball.

I’m surprised he didn’t consult the VAR monitor in the end, but it was ultimately his interpretation of the play and they weren’t about to question it due to it not being clear/obvious.

My opinion on this:

Completely deserved penalty, clear foul in the box.

As for the red, I believe it’s harsh, but not unreasonable. In the replay, to me, the goalkeeper makes his mistake and then tracks the ball the whole way, ultimately fouling Morata as it falls and he pokes it away. In my opinion, he attempts to play the ball and it should be a yellow, but I understand why it was given as playing the man and deserving of a red.