Right, but didn't Moyes sack, or technically order the firing of, the assistant coaches who served under Sir Alex Ferguson, arguably the greatest manager in the history of all sports and who won multiple PL and CL trophies?
The point is that Moyes took a sledgehammer to an organization which had just won a PL trophy, whose squad undeniably needed some refreshing...but Fellaini was in no way a freshening up of the squad. The panic buy of Fellaini fundamentally upset the chemistry of the squad by becoming the long ball target man. As a desperate roll of the dice in stoppage time, bringing on Fellaini was a fine option coming off the bench, but Moyes relied on him as a regular starter to serve in a role that he was wholly incapable of serving.