The Toffees have now won just two of their last 13 games in all competitions, losing seven of those, and are in the bottom three due to the humbling suffered at home on Sunday.
"The team is underperforming. It is in a difficult situation mentally," the Dutchman told BBC Sport.
"That is what we need to change. I still believe I can change the whole situation but everybody knows how it works in football. That is all I answer about this situation so ask somebody else.
"It is how you look to the situation. If you start to think negative then maybe there is no solution. I am not like that."
Everton turn their attention to the EFL Cup in midweek against Chelsea at Stamford Bridge, before then travelling to Leicester City in the Premier League next weekend.