He never really got a sustained spell in the squad, and that was the main problem. There were games where he defended well, when we were deliberately playing a bit more conservatively. I think his main problem was his tactical naivety, often being caught out of position and not being able to get back. And being in and out of the squad never allowed him to build up that relationship with the other players.
But it's definitely possible that he was just crap, we'll probably never know. Let's be honest, there were plenty of times when Cancelo was a liability defensively too, but he had the creativity going forward to make up for it, whereas Mendy's delivery consisted of him rocketing a ball across goal at the speed of sound every time. Looked great when it came off, but more often than not, no-one was getting on the end of it.