It was a clear foul, with no effort made to play the ball, denying a goal scoring opportunity. It doesn't matter if Carvalho was covering, as two City players would have been through, and would clearly have had an advantage. In any case, it's gamesmanship to break the rules thinking all you will get is a yellow, and hardly in the spirit of the game.
It does matter. He has to be denying a clear goal scoring opportunity. Ricardo Carvalho and Jose Bosingwa between you and the goal makes the opportunity anything but clear.