Firstly, although I'm sure that United fans are annoyed by this performance, I wouldn't draw too many conclusions from an end of season match in which United had little or nothing to play for. It's common to get this type of display, I wasn't surprised, especially with a big match looming, and some of them probably knowing that they won't be at OT next season. The FA Cup final will decide their season, if they win it then it has been a decent season in which they've got into the CL on merit, probably finished 2nd, and won a more important trophy than the ones they won last season (IMHO).
Secondly, it's not just the players you listed, it won't be long before Mourinho gets rid of Martial and Rashford because he doesn't trust young players, and never has. He's already hammered Shaw in public God knows how many times. If you're not careful then by the time he leaves (probably at the end of next season, if we are to pay heed to historical precedent) then you will have one of the oldest squads in the PL.
I believe that Guardiola would be getting more out of Martial and Rashford. The reason I believe this is that Sane and Sterling have massively improved over the last two years. Look at Mourinho's signings. Mkhitaryan has already been and gone. Ibrahimovic has been and gone. He was only ever going to be a stop-gap signing, whereas Guardiola signed Jesus, who is probably worth £60-70 million now. Pogba has been extremely disappointing. There's already talk of loaning Lindelof out, and he couldn't even get in the team this evening! I thought Bailly had done pretty well, but he's been frozen out recently, and there is talk of him leaving. Sanchez and Matic have done okay, no better than okay. Lukaku has maybe been a 7/10 performer, but I would say the jury is out on whether or not he fulfils his vast transfer fee.
That's not a great record. At some point Mourinho has to accept some culpability for this, rather than people just saying "there's nothing we can do about it because City have loads of money, and they have a better team than us". 12 months ago, very few United fans would have swapped de Bruyne, Sane and Sterling for Martial, Rashford and Pogba. Now it looks like an absolute no-brainer. That's because, in my opinion (and not only my opinion), Guardiola has vastly improved several players.
I am not saying Mourinho is a bad manager, that would be ridiculous, and it hasn't been a disastrous season for United by any means. But I would question the way he handles and coaches players, because he seems to fall out with them right, left and centre, very few of them seem to have got better, he's particularly suspicious of young players, and several of his own signings have either gone, or they're out of favour. Between the money he has spent, which is considerable, and the fact that you had three of the most promising young players in the world in de Gea, Martial and Rashford, you would think he would get a half-decent side out of it.
I'd also like to mention that you began the season like a train. So there are good performances in this team. Since the rather insipid 0-0 draw with Liverpool, you have won 18, drawn 4 and lost 7 of 29 PL games, for a points return of 58. And you scored 21 goals in your first 7 games, averaging 3 per game, since then 46 in 29, averaging just over 1.5 per game. That's why people are getting frustrated in the match threads, because it feels as if the team has gone backwards, even this season.