Let's start with the obvious reason that has nothing to do with transfers: United will be playing in the Champions League this season, after sitting the competition out last year. Every year, the team that loses European football makes a huge leap in the Premier League. You don't have to look any further than Liverpool in 2012-13, almost challenging for the title thanks to no midweek distractions, just as United themselves surged to 3rd last year. In the subsequent Liverpool season, the addition of the Champions League had the opposite effect, and it was a big factor in their sixth-placed finish last campaign. United will also suffer from playing those additional games, especially since they also can be expected to play more than one Carling Cup game this season (the mighty MK Dons having smashed them 4-0 in the 2nd round last year). Just as I think not having European football will really help Everton this campaign, I think having it will hurt United's chances at maintaining a top 4 spot, let alone challenge for the league.
The second reason is that United's transfer plans seem to be focused in all the wrong places. Let's take a quick look at United's numbers for the past season:
|Off Eff||Def Eff||Possession||FR||Opp FR|
As you can plainly see, United's strengths were in keeping possession of the ball, finishing their chances, and having David De Gea in goal. Their weaknesses lay in limiting opponents' chances and in a striking inability to create chances given their possession advantage. Yet their two signature signings, Schneiderlin and Schweinsteiger, are both holding central midfielders coming from systems that prioritize possession, duplicating what they already had. Darmian will help defensively certainly, but Antonio Valencia had a quite underrated season as a right-back last year, averaging more interceptions per 90 minutes than Darmian and posting a higher tackle success percentage. Only Memphis Depay is an addition that really helps out in one area, as he is the sort of creative player their squad is crying out for, but they still haven't addressed their biggest position of need: center-back. Even if Otamendi or Ramos comes in and is an instant success, that still leaves Smalling and/or Jones playing major minutes this season, something which as a City fan makes me very happy.
You'll notice that so far I've only mentioned the arrivals. But it's the potential departures that should have United really concerned. David de Gea is the obvious name here. If there's one game that was United's season in a nutshell, it was the 3-0 home win over Liverpool. Controlled possession, conceded more chances than they allowed, but won comfortably. That was because David de Gea was a god last year, repeatedly saving the skin of his terrible defense. If his long-mooted, saga-of-the-season-if-it-weren't-for-Sterling transfer to Madrid goes through, United will have lost their best player from last term. They have also let Van Persie leave and Chicharito is sure to follow, meaning for now they only have Rooney and the youngster Wilson as recognized strikers. There are rumors of interest in Cavani, but that would likely signal the exit of Di Maria to PSG. And if Di Maria was to leave, not only would they be taking a huge loss on him after just one year, they would lose their top assist and key pass man (per 90 minutes), making their chance creation even worse.
Definitely transfers yet to come can still affect things, as we're nowhere near the close of the window. But people seem to be viewing United's moves based on the premise that United were close to contending last season, when I don't think it is at all clear that they were. United did finish fourth in the table, and even had the fourth best goal difference. But their Shots on Goal Difference was only 6th, and I think that is more indicative of their true talent level. Consider that last year, Ander Herrera and Juan Mata combined for 15 goals...on 19 shots on goal!!!! That's a finish rate of 79%, and given that the league average was 28% last season and neither Herrera nor Mata is a world-class finisher, this is completely unsustainable (for reference, the duo combined for 11 goals on 31 Shots on Goal the season before). Put them at league average Finish Rates and that drops United's goal difference 10 goals, putting them behind Southampton into 5th on Goal Difference. When you factor in the bonus of not playing European football, I think it's clear that United were not necessarily better than Southampton, Spurs, or Liverpool, and in fact were quite lucky to make it to the 4th spot.
I can certainly understand that the determination United has shown in the transfer market has resonance for United fans given David Gill's baffling inactivity in the few windows prior. However, I really can't see how these signings (and potential departures) make United a better team this year, despite the fact they weren't a great team last year. Apart from Depay, the signings they have made so far don't help cover their weaknesses, while looming departures and simple regression to the mean will likely reduce their strengths. The media and United fans may be showering Gill and Van Gaal with praise now, but based on last year's results and this transfer window, I don't think we'll have to listen to it come the end of the season.