The England international said that he was "disappointed" at being placed in midfield and admitted that this affected his performance on the pitch.
"Everyone at the club knew where I wanted to play and I think that's why I was disappointed," he 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 OK, 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."
Rooney was strongly linked with a move away from Old Trafford before the transfer window closed, but the forward insists that he has felt "settled all summer".
"To be honest, I've been settled all summer," he said. "I really have been happy all summer. From the outside looking in, it might not have seemed like it, but I have been settled.
"My wife had a new child and I'm just delighted to be back playing and injury-free. I've got my head down and worked hard.
"I've been focused and I've been wanting to prove to people that I am a top player. That's what I've really tried to do."
Rooney is expected to feature for England in Friday's World Cup qualifier against Montenegro.