Mental. I don't think it's hyperbolic to say that he's a HUGE reason for our defensive record last season - and not even for his saves. It's about what his presence does for those in front of him. We've seen, all to frequently, in recent years what having a fragile keeper can do to a backline. Coppers persona is so very good for a keeper he exudes calm and confidence that bleeds into everyone in our defensive third.
If it's so easy to get a 'decent championship keeper' explain, Wes, Grbic, A.Davis, Ramsdale, Blackman, Verrips, Olsen, Amissah, Moore. Beyond Wes, who was decent but NEVER inspired confidence like Copper does, our recent history with keepers is doggers. Even Henderson IMO, while outstanding for us at the time, wasn't the complete package that Cooper is.