True. One thing Fergie always had was reliable grafters to form the backbone of his squads. He knew a guy like Butt or Fortune would give 110% in a match against weaker opposition and this allowed him to manage his players better. It was Rio Ferdinand that said Fergie would weeks in advance know who he wanted to play against upcoming opposition and rest and rotate his players as required.
I think the bold says it all.
Players who give 110% against a club they know are smaller, against a team less powerful, against players they know are not as skilled. So as a Burnley (last team to beat us in the PL, right?) would come to OT knowing they are running into a set of warriors on the field, and if the hassling, harassing, outmuscling and outscoring doesn't work for the first 60 minutes, then you'd have to deal with a fresh paul scholes, OGS or Giggs coming at you relentlessly for 30 mins with the full knowledge that they are 90 minute regulars.
No wonder we beat some of the teams before they even arrived.