We saw how well it went with LVG.
I doubt Faria is so "good". Try to apply your own logic here. If Faria is so good, why nobody else want him? Why does nobody offer him a promotion to management etc? Clearly he is only "good" when he assists Mourinho.
Of course the person employed by MU must be good at what he does. But here is the thing, a person who is loyal and has good skill is something different entirely. It can not just win trophies, but rather create a true dynasty, legacy even. If Mourinho and Giggs win PL with United, there is no comparison even which of these two occasions will be better received by fans. Also you look at the greatest successes and in many of them managers were connected to the club before. Like Anci, great manager, had good Real team, good Chelsea team, good Paris one. Was a success in every team, likely will continue that in Munich. But his greatest team? No doubt Milan that won to CLs, and was in 1/2 for 5-6 years or something like that.
I have no idea if Giggs will ever be United manager or will he succeed in that role. But the club understands that he does have a United's pedigree, that is why they are desperate to keep him.