I think it will be a bit like looking back on Tommy Docherty - secured promotion back from Div 2 to third in Div 1 and as far as FA Cup final against Southampton, so successful compared to O'Farrell but personally highly unlikable and inclined to backstabbing. From the moment Mourinho arrived, to me it looked like an appointment for short term success - and League Cup and Europa League were big successes in the context of where we were since SAF's retirement - but long term trouble.
Zlatan was great entertainment value, but clearly not a solution to our longer term needs. His transfers in were mixed - Lindelöf, Bailly, Fred, Matić and Dalot are all still here, with varying degrees of success, but the ones who aren't (or not actively contributing) have been expensive failures. For all his claims that he supported youth, his handling of Rashford was almost deliberately equivocal and his only promotion from the second team was McTominay - his one notable success - who was used for his height as a kind of alternative Fellaini when the latter was unavailable. During the time he was here, the youth teams lost key members of the coaching staff and the second team was relegated. Youth players were constantly denigrated as "kids" ("This is Manchester United, we don't play kids" - !!!) and when he took some of the "kids" on pre-season tour he couldn't even remember their names.
At the time we appointed him the risk was that we would get a couple of years of relative success followed by a big falling out and a big pile of crap to sort out afterwards, and that's exactly what's happened. We are still sorting it out now, although it's not all Mourinho's fault, more a consequence of trying to replace a manager who became the beating heart of the club for 26 years and struggling to find one who would take it in a direction that it felt comfortable with.
So Mourinho will go down as part of that process, which has taken too long, but the only positives from his spell will be what you would expect of him - a couple of pieces of quick silverware, and a decent second season, and even that was 19 points behind City. He was never, ever going to be the man to rebuild the team and that became evident in his third season.