Law scored a total of 237 goals for the Red Devils during his time between 1962 and 1973.
"When you see some players in and around the penalty box, you don't think they'll score," Law told Inside United. "But, with Rooney, you never think that. We always dreaded playing against Jimmy Greaves when he was at Spurs because you knew that if he was in and around the area there was a fair chance he'd score a goal. And Rooney is in that category.
"Along the way, I obviously beat someone else's total [to get to 237] so it's something that is always going to happen in football. I was happy to beat someone else and it's inevitable that Rooney will beat mine.
"I said when he first joined the club that if he can steer clear of serious injuries, he'll be the record goalscorer for Manchester United, there's no doubt about that."
Law was adjudged as the European Football of the Year in 1964.