The 31-year-old has had limited first-team opportunities over the last couple of seasons, and claims manager Sir Alex Ferguson did not make good on promises to give him more chances.
He told The Sun: "We talked 10 times, he told me there would be a place for me but I stayed on the bench.
"My time at Manchester United is running out. I no longer feel like a valuable part of this team.
"I think I did well in the few opportunities that I received.
"I am a little frustrated by the way this happened, I do not think I deserved it.
"But I have dignity and I stopped going to such meetings, it is clear that I'm leaving United. It's obvious that I have to leave.
"I'm looking for a new place now. I know I can still play at the highest level."