This is when it matters the most. Think of end of April, you have a massive CL semi final coming up midweek, and potential title deciders every weekend in the Prem. This is what is expected of you when you play for Manchester United, you're expected to be playing high pressure, high stakes matches. That's why these guys are getting paid so much.
The last four/five games of the season should be huge. Ole will be learning a lot about his players right now, just as much as he would have learned during the dark days. Which players need a break, which players have the will to push through. Of course there needs to be man management, some rotation, but I just can't see Ole fielding a second string team in a FA Cup semi final against Chelsea in Wembley.
Think of it this way, you say to your squad; we've got another gruelling week ahead, but at the end of the line is CL Football and potentially two trophies on the horizon. Are you tired? Do you need a rest? Fair enough, he might know when to pull them out of the squad, but I sure as hell hope fatigue is the furthest thing from our players minds right now.