"If you've got a player in his category than money doesn't come into it," he said after accepting a Nationwide Lifetime Achievement Award. "If United don't give him a new contract, other clubs will get him. You need to keep the best players.
"I heard (assistant manager) Carlos Queiroz say they wouldn't sell him for £96million. That's a good tactic. Players of that quality are few and far between. Figures don't mean anything if the players are better than the others in your team. Sir Alex Ferguson won't be selling him."
With Ronaldo plundering goals at will, Law will surely see his United record of 40 strikes in a season surpassed.
The 23-year-old, who last month eclipsed George Best's record of 32 for goals by a winger in one campaign, has as many as seven games left to usurp the former Scotland striker.
Law has little hope that it will remain intact at the end of the campaign and admits he will feel a pang of sorrow at seeing his mark overtaken
But he is full of admiration for a player who he compares to his former United team-mate Best.
"It will be sad but records are there to be broken. The way Cristiano is playing I'd say that's fairly certain," he said. "He's an excellent, excellent player. I thought he might have a problem coming back from the World Cup after the problem he had.
"But he's come through that and has become far more mature. This season he'll probably win all the awards. He's scoring goals like nobody's business and that's why United are at the top of the Premiership. He must be rated among the best players in the world.
"United can do a Premiership and Champions League double this season and Ronaldo is a very important part of the team. George Best was similarly prolific. As a winger who scores many goals George could be placed in a similar category.
"They both have skill in abundance so it's very difficult to judge who is better. Who'd like to say? They're both terrific players."