"My time at Manchester United is running out," the 31-year-old Bulgaria striker told bTV on Sunday. "I no longer feel like a valuable part of this team."
The Premier League runners-up took up an extra year's option on the former Bulgaria captain's contract in March despite him slipping down the pecking order at Old Trafford.
"I think I did well in the few opportunities that I received," he said. "But I am a little frustrated by the way this happened, I do not think I deserved it."
Berbatov, who is Bulgaria's all-time leading scorer with 48 goals, joined United from Tottenham Hotspur in 2008 for £30.7 million.
He was first-choice at Old Trafford until the latter stages of the 2010-11 season, when he fell out of favour despite being the Premier League's leading scorer.
The former Bayer Leverkusen striker was famously dropped for the Champions League final, with fringe player Michael Owen taking his place on the bench for the defeat Barcelona.
Berbatov stayed to fight for his place but, despite Javier Hernandez losing form and Owen barely seeing any action through injury, he is still mostly used as a substitute.
"I've talked 10 times or even more with the manager and he always told me there would be a place for me in the team. But after that I continued to stay on the bench," he added.
"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."
Berbatov has been linked in the media with several clubs from Italy, Germany, France and Turkey, but he said he would not join United's city rivals Manchester City.
"I would never join Man City, I am a loyal person after all," he said. "I had the opportunity to sign for City in the past, but I chose United."