There was a pretty similar discussion in the Alexis Sanchez thread a couple of days back. So I guess I'll repost some of my comments there:
Man Utd, before and after Ronaldo:
-- 2006/2007 - 1st, 83 scored, 27 conceded
-- 2007/2008 - 1st, 80 scored, 22 conceded (Ronaldo's best season)
-- 2008/2009 - 1st, 68 scored, 24 conceded - Ronaldo with 18 goals, Rooney with 12, Berbatov with 9, Tevez with 5
-- Ronaldo and Tevez leave in the same window
-- 2009/2010 - 2nd, 86 scored, 28 conceded - Rooney (26), Berbatov (12), own goals (10), Nani (4)
-- 2010/2011 - 1st, 78 scored, 37 conceded - Berbatov (20), Hernandez (transfer in; 13), Rooney (11), Nani (9)
I'll add some data from the last 3 seasons:
-- 2011/2012 - 2nd, 89 scored, 33 conceded - Rooney (27), Hernandez (10), Welbeck (9), Nani (8), Berbatov (7)
-- 2012/2013 - 1st, - 86 scored, Van Persie (26), Rooney (12), Hernandez (10)
-- 2013/2014 - LOL
Replacing a world class player? Can be done, has been done. Spurs didn't manage to last season, but the Mancs succeeded. Arsenal lost Van Persie but didn't falter as badly as predicted because Cazorla, Podolski and Giroud chipped in with 11 to 12 goals each when they arrived and made up for the missing goals while Walcott stepped up and got 14. Nobody would've put these guys on the same level as Van Persie but the team ended the season with just two goals fewer. The next season, when the production from these guys bar Giroud all dropped due to injuries / loss of form, etc., Arsenal got 10 goals from Ramsey and some additional output from Giroud.