The Crystal Palace defender was last night undergoing a United medical at Carrington.
The 33-year-old DC United striker has sent a message to the defender and outlined three things he must do in order to succeed.
"Enjoy it. Play with no fear. United's a fantastic club," Rooney told Sky Sports.
"One of, if not the best football club in the world. You have to embrace it, embrace the culture of United. Learn the history and do your best."