Although a prolific goal scorer is important its just as important to have goal scoring midfielders - Gerard and Lampard were top scorers for Liverpool and Chelsea, with 20 goals each in all competitions. Both teams won a trophy. Arsenal with Henry scoring 33 goals in all competitions won nothing.
As for Saha and Rooney - Saha scored 15 goals in just 30 matches, Rooney scored 19 goals in 48. So its quite possible for both to get 20 plus goals in all competitions, especially if Saha can remain injury free.