I don't think it's a matter of form.
No matter how bad a players form gets, they're always physically around the same level. Diers physical level is the issue right now. Almost every player in the premier league can run off the back of him and there's nothing he can do to stop it. He's slow to turn, slow to pick up speed and even his top speed is quite pedestrian.
0:41.
1st goal vs Bayern. He's the SECOND HIGHEST PLAYER up the pitch, pressing a central defender. The central defender takes one step to the right and plays a simple pass out of trouble. 1. Why is Dier even there? 2. Dier is so slow that a single step takes him out of the game. 3. He never manages to get back to defend after this, as he's too slow and lethargic. Eriksen and Lo Celso are both deeper than he is.
2:46
2nd goal vs Bayern. Starts off in a decent position, but isn't actually marking anyone. 2:52 onwards is a catastrophe. He goes to press a player, does absolutely nothing to stop a pass-and-go. Then he stands still like a lost puppy, jogs like a human slug towards Muller who simply taps in the ball.
Slow of mind, slow of body. Players will purposefully let him man-mark them in the box, because it creates space for other places; he isn't actually stopping anyone. The premier league has moved on from big bulky defenders, the best midfielders in our league tend to be very small, quick, energetic players such as Kante, or extremely technical players such as Shelvey or Grealish. End of the road for Dier imo.