Rooney is being warned he must get used to life in Van Persie's shadow.
Ferguson said: "Van Persie is, without doubt, an up-front player.
"I think the players are now starting to realise what a good player he is."
Dutch striker Van Persie has hit 12 goals in 18 games for United but Rooney's tally is three in 14 matches.
Ferguson said: "I think he's going through a period that all strikers do.
"He's not scoring but it will change once he starts to score regularly. It will become better for him."