The problem is that Dier isn't doing anything in this team. Dier was always a bit slow off the mark and slow to turn even when playing at his best, but was a Gazelle back then compared to his mobility now. There is no point in having him in the team as he isn't being positionally disciplined (we constantly seem to have acres of space in front of our defence for opponents to get on the ball) and it looks as though he can no longer run. Far more worrying than the numerical overloads that Chelsea were creating on the flanks was the number of times they were able to carry the ball through the centre unchallenged. Typically every time this happened you would see Dier casually jogging back (or perhaps that is as fast as he can run now?) about 5 yards behind play. I wouldn't have any issue in having a player as technically limited on the ball as Dier playing in the centre of our midfield if that player was actually good defensively. However Dier's defending now seems to be no better than his attacking contribution. It is pretty much like playing with 10 men at the moment. All I see Dier doing is taking the ball off of one of the centre halves and passing it back to them.
Don't get me wrong, I don't think that Sissoko should be in this team either. I think that (in order of preference) any of Ndombele, Lo Celso, Eriksen or Winks should be playing the second central midfield role. It's just that I would jettison Dier and replace with Sissoko temporarily until we can sign a proper defensive minded midfielder.