All the managers who've discarded him are blind or on the sauce or crap?
They work with him day to day, they know more than you and me (except Martinez) I trust them.
One of the three managers who discarded him had a sensible reason to do so - Sevilla were a good side, were successful and there wasn't any real reason to upset that just to accommodate someone who was on loan anyway.
Martinez bought him and then wouldn't play him regularly even though we lacked creativity or pace and were in relegation form; Koeman wouldn't play him even though we lacked creativity and pace and were in relegation form, and even when he did start him (against Watford) he dropped him straight afterwards as if that was down to him, and not defending of the clown kind.
If he had been given a run in the team and failed, you would have more of a point. In fact there has only been a couple of occasions on which he has had a run in the team (in his loan season up until he got injured, and at the start of last season) he was one of our best players; not to do it (and worse, to pick Lennon and Valencia instead) is just daft and any manager who does it should really be questioned.