van Gaal fecked it with the clear out.
In his first summer we lost Vidic, Ferdinand and Evra. Vidic he couldn't do anything about, and Ferdinand was over the hill by that point, but I losing him as well meant we lost all of our experience in the centre of defence. Losing Evra on top of that just caused a massive void at the back, that van Gaal could only cover for by playing dull as feck, defensive football.
As well as the above, he also decided to loan out Nani and Hernandez, which meant we'd lost options in attack that we knew could perform in the Premier League, even if they weren't necessarily starting quality. A similar story with Welbeck, and to a lesser extent Kagawa, who although still a bit unproven in the Premier League, was at least more experienced than Di Maria and Falcao, who were the only remotely attacking signings.
We also decided to let Fletcher go in the winter window, which left us with just an aging Carrick as an experienced central midfielder, and similar to the attacking players above, while not starting quality, he let both Cleverley and Anderson go that season.
I'm not saying we should have kept all of these players, but to let 10 players from the senior squad leave in one season is insane management, and he should never have been allowed to do it. This is without discussing the departures of Michael Keane, Zaha and Buttner in that time too.
The fact that this was followed a season later with the departures of Rafael, van Persie, Evans and Janzuaj is even more insane, not to mention that Falcao wasn't kept on, and Di Maria sold after a single season.
Massive clear outs like the one van Gaal was responsible for are FM fantasies.