He now concedes that he should have stuck it out at United and waited for an opportunity to impress.
Ahead of a 2014 World Cup qualifier against Scotland on Saturday, the 25-year-old forward told the Daily Record: "It all happened really quickly as I played for just 18 months with Partizan Belgrade.
"We won the league and the cup and there was a period when it seemed like I was scoring in every match.
"I had grown up as a United fan so to be working for the man who had brought so much success to the club was just unbelievable.
"However, there is a big difference between the Serbian and English league and when I arrived in Manchester I discovered I was not yet ready for that level.
"I spoke with Sir Alex many times and he kept telling me I had quality and just had to keep working hard.
"But I wanted to play more games so went to Cologne in Germany on loan and had a great six months there.
"When I came back to Manchester I was expecting to play more games for United but it didn't happen.
"In the end I wasn't sure I was going to get the chance I felt I deserved so decided to change club. Even now I am always thinking about that moment. Maybe I shouldn't have been so quick and should have worked longer to show Sir Alex.
"Maybe I should have stayed and fought for my place. This was Man United after all."