Every action has an equal and opposite reaction...it doesn't work like that, managers are not infallible. Mourinho has gotten plenty of transfers wrong. United does not exist in a void, City is competition, competition that has been trumping us for a couple of years now.
The manager handing is shortlist 2 months ago is relevant in the context of people getting anxious because direct rivals have already tied up a very good player with a high ceiling and are seemingly pretty aggressive in the market, meanwhile we are being linked with pretty average players. Does it explain some of the reactions? yes, it very much does so. are those reactions justified? you can argue that I guess.
Yes we won 2 trophies but let's not kid ourselves, they aren't the major ones or at least the ones we should be gunning for. secondly, we fell short in the league, badly. thirdly, our performances haven't been consistently great for like 2 or 3 months now, so yes, signing players is pretty important unless you wannabe stagnated or even worse. We are already behind our competition in the league, let alone in Europe.
It's great that you are enjoying the europa win, but it's also ok for others to voice their opinion/displeasure with the type of players we are being linked to, I mean, if it was all "I trust the manager" type stuff, what would even be the point of a forum?
When people start getting upset others will start calling them out about it. I think both sides are fair but there just seems to be an outrage culture online so things always become extreme in nature.
I think people are on edge because winning Europa and getting into the Champions League was meant to be the turning point, the factor which ended our run of mediocrity since Fergie left. We desperately need to stop being a team fighting for a CL place and actually challenging for the title. However so far we've been linked with a pretty uninspiring list of players while City are snapping up the hottest talents within days of the season ending. I don't think this is what anyone expected when we won Europa and I think people are extremely fearful (consciously or subconsciously) of going another season fighting for a top 4 place and maybe even missing out. I don't think I could face another season of not qualifying for the champions league.
That said it's early days and some have more patience and a calmer demeanour so are happy to wait and see who we get. But the days are slow and for some it's harder to cope with (me included)