Richards believes the two clubs are now neck-and-neck.
"After winning the league you want to come back and retain it," he told City's website. "I was missing, Vinny was missing and Sergio was missing and Yaya was away for five weeks. People think you just need a good starting eleven, but you need a strong squad to challenge for places and come in and compete when necessary.
"Last season we were a bit short on numbers and in the end it showed with the massive gap with United. Their squad has become amazing now and they've got just as good a side as us now so it'll be tough this year but we're looking forward to the challenge."