The fact that he rates Cuco Martina should be enough to sack him.
Has anyone noticed that our opponents leave that side of the pitch open every time he plays? The reason Cuco gets on the ball so much is because that's what the opposition wants. Nothing good happens after he touches the ball early in a build-up. There was one point in yesterday's match where Calvert-Lewin did some great work to retain the ball in the middle of the pitch and, as he was dribbling out to the left, Martina was totally unmarked in space. Calvert-Lewin held the ball for as long as he could have looking for another option before finally passing him the ball.
Cuco seems like a good guy and all and the Koemans clearly like him, but neither of those are good reasons for signing a player. If the club is truly ambitious, there are certain baseline requirements for player quality and he plainly does not meet them.