The England star, who played second fiddle to Robin van Persie up front in 2012/13, reportedly wanted to leave United this summer, but the club warded off firm interest from title rivals Chelsea and Arsenal.
And despite persistent speculation over his future, Rooney has started this season well, netting five goals in eight games.
"Everyone at the club knew where I wanted to play and I think that's why I was disappointed," Rooney told reporters.
"I got told to play in midfield and I didn't want to. I just think there had to come a point when, for my own career, I had to be a bit selfish really.
"I actually felt when I played in midfield I did okay, but I didn't want to play there. I've had no problem in the past playing out of position, but I felt I deserved the right to play in my position and that wasn't happening.
"I think, naturally, I was a bit disappointed and maybe that affected some of the games I played. I know myself that last year wasn't my best season but there were times when I was playing in different positions.
"I didn't feel I got a consistent run of games up front. Sometimes when you're not playing in one position all the time it's difficult to adapt."
Rooney went on to discuss the differences between Ferguson and the Scot's successor, compatriot David Moyes.
"It's a bit different in terms of David Moyes and his staff coming in," he continued. "The training is different, there's a different style, a different way of thinking about how he wants us to play.
"Honestly, the training has been so hard. We've done a lot more running. Long running, quicker running, sharper running.
"I feel that's really benefited me. I feel as fit as I ever have done. I had a couple of injuries pre-season but I feel great and in good shape. It's a mixture of things, but I am happy and I am enjoying my football."
Regarding his future, he added: "You can see I'm playing and I'm happy in my football. I'll have more discussions with the club and we'll see what happens from there."
Rooney is contracted to United until the end of next season.