The 24-year-old joined Everton from Burnley during the recent transfer window following an impressive campaign that saw him earn his first England caps and a nomination for the PFA Young Player of the Year award.
Keane, who came through the ranks at United but only ever made one Premier League appearance for the club, revealed that the Red Devils wanted him back at Old Trafford this summer, but that he chose Everton due to the likelihood of getting more regular game time.
"[United] were in touch with my agent and showed a lot of interest. They wanted me, but a few things happened and I thought Everton would be the best place for me to come. I thought I would get a lot of game time here, more than I possibly would at Manchester United, and that was a massive factor in my decision," he told The Times.
"To be honest, having been at Burnley for three years, I wasn't a fan of the club like I used to be. I wasn't as big a fan of Man United as I had been, so I had to sort of put that to one side and just think rationally about what would be best for me as a footballer. My family are all Man United fans so it was not an easy decision. My dad would have been delighted if I had signed for United. But he is glad I have come to Everton and not Liverpool, put it that way.
"Of course [people will question it]. United are one of the biggest clubs in the world and it is not an easy decision to make. I thought about it all throughout the summer and a bit longer. I spoke to my family, friends, agent, I got advice from people I used to work with at United. Loads of different people. In the end, I came to the decision of joining Everton and I am thankful I did that because it is going to be the best thing for me in the long run.
"We have had games where we've played well and a few games where it hasn't gone so well. But I am young and learning and that is going to be good for me. While they [defeats] are not enjoyable, you go through them and learn and it will make me a better player. You have to look at what is right for you as a person. I have done that throughout my career."
Keane is expected to come up against United when Everton travel to Old Trafford on Sunday.