The 34-year-old Manchester United defender called time on his international career on Tuesday after winning 81 caps, stating he was determined to concentrate his efforts at club level.
Ferdinand has not featured for England since a Euro 2012 qualifier in June 2011 against Switzerland but was recalled by Roy Hodgson for the World Cup qualifiers against San Marino and Montenegro in March, before controversially withdrawing due to a "pre-planned fitness programme".
Mills, who played for England between 2001-2004, said: "I don't think it's a great surprise after what happened last time.
"Rio wanted to be in the squad and then realised with all his rehabilitation he wasn't going to be able to play back-to-back games which ruled him out of tournament football for England."
Hodgson will unveil his England squad on Thursday for the end-of-season friendlies against Republic of Ireland at Wembley on 29 May and Brazil in Rio de Janiero four days later.
Mills added: "He would have made himself available for the next friendlies and the games in the summer but he probably looked at what happened last time and thought the chances are that Roy Hodgson might not select him.
"So maybe Rio's done the right thing and retired before there was a chance he doesn't get selected."