Dont judge the goal keeper yet because you watched a compilation of his mistakes as a GK. Every player has their best and worst moments in football. This shouldn't be what the fans should focus on.
I also watched the video and my conclusion is this, he may not be what Manchester United want at the end of the day but I am sure he is better than Andre Onana.
He's young but don't judge him by a comps of all his career blunders, I also watched some clips not a big fan but for the benefit of doubt the United fans needs to back him up. I'm surprised they ended up with such a younger goalkeeper in a market where both Donna and Martinez were literally put on for sales. Went for both of them initially and ended up with inexperienced Lammens on deadline.
Emil Martinez signed a new 4 years contract after the transfer drama, Villa wanted him to stay, it obvious he was the one pushing for a deal.