The Red Devils' defender turned down an invitation from Roy Hodgson to re-join the international ranks earlier this year.
The 34-year-old could have been in England's squad for the World Cup qualifiers against San Marino and Montenegro.
But Ferdinand told the Sunday Sun: "Basically, I could play for another three or four years or play two or three games for England and be finished in a year.
"It wasn't a contest. I couldn't sacrifice my fitness for the sake of a few more England caps.
"It would not have been fair on United, it would have been a disservice to the club and to England."
Ferdinand has won 81 international caps, the last one arriving in June 2011 against Switzerland.
Recalling his chronic back injury, he added: "Two or three years ago people were saying I was finished and I remember those dark days.
"I was in a depressive state, I will admit that. They weren't easy times and I couldn't play to the highest standard.
"You think negative thoughts. I had 18 months of being injured on and off."
Ferdinand also revealed he considered quitting the game when Sir Alex Ferguson announced his retirement.
He said: "Sir Alex leaving did make me think: Is it time for me to go too?"