"Playing for Manchester United was a big, big experience and I enjoyed every minute of it," he told BBC GMR.
"But there came a point where I had enough. The whole thing was becoming too much for everyone, including my family.
"I am enjoying a game every week now. When you play every three or four days, with the travelling as well, you are on auto-pilot.
"Eight years is a long stretch with very little time to recover. I couldn't do that today."
Subscribe to the Red Issue magazine on line:
UK Subs £18.50 for 10 issues
Euro Subs £22.50 for 10 issues
ROW Subs £29.00 for 10 issues