I don't believe it is always down to effort but his inexplicable trust in certain players that aren't worth the stupid privileges.
Pereira hasn't been as bad for the U21s as Rooney has been for the first team. When he was out of favour, he looked like he knuckled down and forced his way into the U20 for Brazil, something that is hard to get in if you don't ply your trade in SA, and if you don't, you have to be a regular in Europe. He had neither so it was clear that he worked hard to augment his natural ability to be considered for selection and I'm sure he'll represent them as a key player at the Olympics in Brazil.
It's all LvG imo. His whole thing is centred around Rooney. When it was clear that Rashford, was the better striker, it was all about shifting things around to accommodate him. My question is, if there were 4/5 Martials and Rashford, would LvG oust Rooney for them, I think not.
First, Martial came to United on the promise of leading the line, he got moved for Rooney. Rooney started playing crap and got injured, Rashford came on the scene, things were yet again shifted for Rooney at #10. Now things have been moved around for Rooney to be shoehorned into the team at CM. I believe that bar experience, Pereira would have added more than Rooney over the period of the whole season and it's not like Rashford or Martial are very experienced.
Pereira is up against a manager that considers his captain undroppable. Even Mata and Herrera don't have a chance to compete fairly with Rooney.