Rooney used to be the man United looked to for their goals, but the England star is being warned he must get used to life in Van Persie's shadow.
"Van Persie is, without doubt, an up-front player. I think the players are now starting to realise what a good player he is," the Sun quoted Ferguson, as saying.
Dutch striker Van Persie has hit 12 goals in 18 games for United but Rooney's tally is three in 14 matches.
Ferguson said about Rooney: "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."