The 22-year-old was Alex Ferguson's last signing at United, however, what started as a dream £10million move soon turned sour.
And the winger admits it was a time that affected his confidence and marred his career as he sat watching from the stands.
He told The Telegraph: "There is nothing worse than not being involved. You are sitting there thinking 'what exactly am I? A footballer who is not playing football?'
"You feel a bit worthless sitting in the stands, watching all the time. You have nothing to speak about with the players.
GETTY
Wilfried Zaha admits he felt useless sitting on the sidelines at United
Eventually I spoke to Van Gaal and he said that right now I wasn't good enough to get into the first team
Wilfried Zaha
"I'm frustrated when I am not playing. My family is frustrated. My friends are frustrated."
But Zaha's career is now back on track thanks to a return to his home club Crystal Palace - a deal made permanent in January.
And the attacker thanks Louis van Gaal for the move, as previous boss David Moyes never explained why he wasn't part of the picture at Old Trafford.
He said: "Eventually I spoke to Van Gaal and he said that right now I wasn't good enough to get into the first team and I put my hands up and thought 'no problem, I will go on loan'.
"I'd rather he told me. Before that it was just rumours and here was a straight answer. I appreciated that and so I came back to Palace."
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