"For a player of his kind of level, we must pay what is necessary," he told German television channel DSF. "Because, for us what is most important is to win the Champions League and he can help us achieve that."
United have reported the Spanish club to world governing body FIFA over what they consider to be attempts to unsettle the player.
And the Madrid boss admitted he would have behaved similarly with such a vital player as Ronaldo, who scored 42 goals in all competitions last season.
"When you have a player of such quality nobody wants to let him go," he added.