Any club taking on a new manager is taking a risk. If Everton had taken on Pochettino when he first came to England can you imagine what the reaction on here would be ?
There are probably no more than four or five grade A managers around and it would be difficult if not impossible for Everton to attract most of those, we are simply not in that league at the moment.
Mourinho, may be a possible exception, because he is currently out of work and clearly wants to stay in England. But, he is prepared to wait for the Utd job where many do no want him rather than come to Everton where he would be regarded as a God.
But, even with Mourinho there would be a downside, in that our carefully nurtured academy would more or less be thrown aside if his past history is anything to go by.
We should not get too fixated on a manager having a bad time at a previous club. Sometimes, it just doesn't work for them. There can be difficulties with established players, the managers style may go against the ethos of the club. There are dozens of examples of good managers failing at a club. Historically, if we go back to Brian Clough at Leeds. More recently when
David Moyes went to Utd he never realistically had a chance.
If a manager learns from the mistakes made it can make them a better manager if they are lucky enough to get another chance.,
