United sit atop of the table by a clear 15 points.
Mancini said: "It's over, but this doesn't change anything because we should do our best from now to the end.
"Every top team, when you can't win a title, you should try to play well, try to win all the games, and we have the FA Cup semi-final.
"We have Chelsea four points behind, Tottenham five points behind and we play another nine games. For this reason it's important to keep working hard."