Fellaini is normally plan B for a van Gaal team but became plan A this season, we've seen he tried to switch it up in the 2nd half. It just didn't work out today.
He isn't helped by constantly having to change our midfield because of injuries, I'm sure he doesn't want Fellaini-Herrera-Rooney as his first choice next season. He has to put Rooney in there because of the 'balance', we lack defensive minded midfielders who can play if Blind and Carrick are out. Fellaini has his strenghts but is a unique player who needs to be used accordingly, we just didn't have the options for that today.
I'm pretty sure we won't see Di Maria in midfield any time soon, if at all. He's much better with freedom, in midfield he would be too restricted so Rooney gets the nod over him since he's better defensively. This is why he already said he wants a new CM, we're very light on the defensive options. As per 'philosophy' he won't put a team out there who's lightweight in the middle because that'd be suicide with our possesion based style.
We couldn't break the wall down today, didn't think it had much to do with tactics. We got close, but not close enough. We'll get beat like this more often in the future under van Gaal in stead of being outplayed, we'll need to become more consistent. Another bad day at the office, but I'm not going overboard because of this.