Koemans tactics and attitude to setting up the team hadnt changed massively from last season. The difference was Lukaku being gone.
Not having a player in your team who can be relied on to regularly score goals is a massive issue and can easily cause morale to drop. Our squad with the exception of Lukaku is not weaker than the previous year.
If you look at just the Brighton and Burnley games, a goal scorer would have likely yielded another 4 points. That alone would have had us at least 11th, which is not a sackable position.