"Robin doesn't need a rest, that's for sure," the Scot told reporters on Thursday. "He's a strong lad with a great physique.
"The goals will come. All strikers have little dry spells - you always hope it ends quickly. It's just part of being a striker."
The Dutchman last scored for United in a home game against Everton in February and, with 19 goals to his credit, has fallen behind Liverpool's Luis Suarez (22) as the league's top scorer.
Ferguson said Van Persie was unlucky not to be credited with the winner at Sunderland last weekend, when his shot took a deflection off Titus Bramble's thigh and was given as an own-goal.
The 29-year-old scored the winner with a late free-kick in a 3-2 derby victory at City's Etihad Stadium last December. United are now 15 points clear of their rivals with eight games remaining.
Ferguson said striker Wayne Rooney and right-back Rafael were back in training after missing United's 1-0 defeat to Chelsea in an FA Cup Sixth Round replay on Monday.
"Hopefully they will be fine by Monday," said the manager.
"Jonny Evans and Nemanja Vidic are not there yet but, hopefully, they will start training maybe tomorrow or Saturday morning. We have a little bit of time on our hands to get most of the troops ready for the game."
