I feel as though we were pretty good at defending balls into the box overall. Dier for me has always been good at that, be it set pieces or wingers easily getting beyond our wing backs and putting a ball in the box. He had some horror shows 1 on 1 and in ground duels, but that's what people remember more than his high defensive stats in other areas.
Dier was supposed to be the meat in a 3 man back line. Romero, Langlet and our midfield 2 let him down too. He doesn't want to go in for tackles because that's not his strength, but opposition targeted that with great success.
Forster conceded more goals per 90 than Lloris. In fact Forster saved 28 of 55 shots faced over 14 games (50.91%) which I believe was the worst in the entire league.
Playing deep doesn't automatically translate to protecting defenders. I would argue it can do the opposite, expose them more and ask more from them. Don't be shocked if Dier and Sanchez for example go to other clubs and play relatively well.
(I do believe we need to replace them still.)