City will head into the new season as favourites to retain their crown but Rooney believes they will not find it easy.
He said: "It'll be a different situation for them, something they've never experienced before but we know about it.
"We know all about it when you're trying to defend your trophy.
"We know that over the years everyone has tried to raise their game when they play against Manchester United.
"Now obviously City are champions they'll have to face that. Sometimes it can be more difficult."