The Netherlands international has not scored for his club since a 2-0 victory over Everton in February after his effort at Sunderland went down as a Titus Bramble own goal.
Van Persie has scored 19 times in 29 Premier League appearances for the Red Devils this season after completing a £24m move to Old Trafford from Arsenal in August.
And Law, who scored 237 goals during a 11-year spell at United, is confident Van Persie will soon find the back of the net again and rediscover his goal-scoring form.
“If you look back through the careers of all strikers you will always have a spell where you’re not scoring,” he told talkSPORT.
“You have a situation where you hit the post, the keeper makes a fantastic or the ball just runs away from you.
“But if you said to most managers around the world, ‘would you like Robin van Persie in your team?’ even during this lean spell, then everybody would say yes.
“Being a striker is a strange position. When you’re scoring goals you go out and think you’re going to score every game.
“But equally when you have a lean spell, you think ‘when am I going to score again’. You have to score sometime though and, once you’ve scored, then you’ll be on your run again.”
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