Hear this a lot, but think it is very wide of the mark.
Moyes, did not lose Rio, Vidic, Evra, and Giggs. He took over a squad that were the champions, admittedly ageing, and systematically destroyed the not only the back room staff, but the team's confidence. Changing for the sake of change and largely playing very conservative football where the players could not string more than 2 passes together. We were outplayed or dominated in nearly every game, even when we won.
LVG took on a much weaker squad, and has let go of quite a few squad players, Welbeck, Chico, Cleverley and so on. So his starting position is significantly different. He has replaced 5 players- all first team players in their own right. For various reasons they have not played as much as one would have hoped. LVG's changes have mostly been due to injuries. The drop in performances has come from the lack of LVG's new blood and therefore our lack of creativity from our existing squad.
When people talk about dire football, I take it they think it is was OK for Fergie's last 2 or 3 years, where for the majority of the time it was very hard to get excited about our play.