Would have started a new thread but there are already several so will stick this in here. His 5 biggest mistakes in my opinion and those that will get him the sack:
Right Back
With Coleman injured, this was a great opportunity for him to play a young lad, an Evertonian, full of confidence from a great season with the Under 23s and World Cup win with England under 20's whose game is about getting up and down the wing and strong tackling. Instead he went for Cuco Martina who has proven to be inconsistent and poor defensively. When he doesn't think Martina is performing he then opts for Mason Holgate. Whilst Holgate is a talent, he looks far better suited to centre back, much like John Stones, who often looked limited at full back as his strengths do not lie in getting up and down the wing.
2 Defensive Midfielders every game
2 of our most influential players last season, who prospered in a partnership with a 3rd player alongside them, Tom Davies. Gana and Schneiderlin like to win the ball back and play 10 yard balls to more attacking minded players. Not only has Lukaku moved on, we now don't have the likes of Barkley or Mirallas to play the ball to so both of these are struggling badly to create anything as it's simply not in their game. Tom Davies should be starting alongside just one of them.
Lack of a true left winger
He started off without playing a winger on either side but has gradually eased Vlasic in to the side who has been a positive. However, the left hand side has been completely ruined by Koeman's decision to push players out of position there. We have 3 options who could play on the left, 2 with pace in Lookman and Lennon, and Mirallas being a 3rd option since he has played out wide most of his Everton career.
Poor balance/shape
This is similar to the above point but there is no balance to the side as we don't play with a natural left winger. This is having a negative effect on so many of the players, starting with the centre backs, but in particular Baines, Gana and Schneiderlin who are struggling for options ahead of them.
Centre forward
Each game he seems to change his mind. One game it's Rooney, another it's Calvert Lewin. For me, if he actually started to play with width, i'd go for Rooney or Sigurdsson in behind Calvert-Lewin. Rooney likes to drop deeper and if we played with width we could cross balls in to Calvert Lewin with Rooney or Sigurdsson waiting on the edge of the box for any knock downs.