Rooney has endured a problematic start to the season after suffering a nasty thigh injury against Fulham which kept him out for just under a month.
Ferguson believes that Rooney is nearing his best form and expects the England striker to "explode" into life in the next few weeks.
He said: "Rooney is fit, he has trained really well. I think he will start scoring soon, once he gets a few games back.
"I think Rooney will explode again soon. It is just the start of the season.
"We know that, his track record tells you that each year he gets better with his finishing.
"I think Rooney and Van Persie will get over 20 this season."
United continue their quest to win back the Premier League title with a trip to Newcastle United this weekend.