I agree with you, in one respect Dembele looks like our best player by miles - he strides around smashing up other players, fending them off, gliding about like a Rolls Royce with a broken steering rack, left then right (no not really, left every time and you fall for it every time)... but is he actually helping us?
Contentious I know, but is his gliding about really what we need? What we need is for an attacking player to get the ball as soon as possible. There is a myth that Dembele draws players towards him and then we exploit the space. The truth is that he beats a man, then lays if off, but by then there is plenty of cover.
In terms of dropping someone, I would drop Dier. He was at fault for the first goal (weak attempted header) and the second goal (trying to play offside, then chesting it to the attacker, then allowing him to score past him).
Dier has been in poor form for a long time now, we should drop him and play Winks. Whatever the formation, if that means a back 4 or Wanyama staying back, that is Poch's job.