Powell was snapped up from Crewe in 2012.
He told the Manchester Evening News: “My time at United was poisoned by falling out of love with football.
“I didn't feel like I was part of anything at the time. I was in limbo between reserves and first-team.
“I remember people trying to find me because I was late to a reserve game, reserve training, first-team training - purely because I didn't know where I was supposed to be. I'd get onto one training pitch and I'd be told, 'no, you're over there this week' and I'd be 10, 15 minutes late and I'd be getting fined. But nobody was talking to me, I didn't know where I was supposed to be."
Powell added: "It was always going to be different when Sir Alex retired. If he wasn't there and I didn't have a phone call and meetings with him, I probably wouldn't have gone to United. So it was always going to be a downhill spiral for me as soon as he left."