The England international scored his 202nd goal for the Premier League champions on Saturday during their 2-1 defeat to West Bromwich Albion, and is now just 47 goals behind Sir Bobby Charlton's record tally of 249 for United.
Law, who netted 237 times for United, is certain that Rooney will enter the record books at Old Trafford.
"I wasn't surprised to see Wayne reach 200 goals for United," Law told United Review.
"I said years ago that as long as he stayed fit he would become the highest scorer and I have no doubt he'll break the record."
United travel to Shakhtar Donetsk on Wednesday in the Champions League.