Rooney was a 10-15 league goals a season man for most of his career with us. So were Cantona, Hughes, Cole and Solkjaer.
Strikers who score 20-25 goals a season are as rare as hens teeth - in the PL era, only Van Nistelrooy did it consistently for us. What we need is a centre forward who can form the focal point of a functioning attack. If he can do that and his 10-15 goals make up part of a 35-40 goal 3 man attack, that's absolutely ok.
The best United attack of my lifetime, between 2006 and 2009, had Rooney scoring 19, 12 and 12 in three seasons, Ronaldo scoring 17, 31 and 18 and then a combination of Solskjaer, Saha, Tevez and Berbatov scoring 8, 19 and 14.