I can say it is almost a certainty. He has been in charge for 11 years. In all that time only Sam Alladyce (when we finished 17th and 11th) has finished above him twice working on a similar budget.
Surely if he were just an OK manager then someone like Martin O'Neill, Martin Jol, Harry Redknapp or one of the other quite highly rated managers would have managed it?
Yet when those managers have finished above Moyes they have only done so by being given a budget much higher than his. When they operate under the same conditions they drop down to midtable.
If someone, just one, manager got teams to similar points totals to Everton on similar budgets then I'd accept it was possible to do. But nobody has, not one, in 11 whole years.