For the most part, I agree with Skyfarer, as usual. Also, I think that SAF team was the last non old firm team to win the SPL too.
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Rijkaard very nearly messed up on a seemingly impossible level in the 2006 CL final, so I'm probably not his biggest fan, but as far as what sort of difference a manager can have...
Rijkaard had almost all the pieces of Guardiola's amazing 08/09 team, but only managed to come third in the season before Guardiola's arrival. (Rijkaard didn't have Dani Alves or Pique... Busquets was in the youth team. Guardiola didn't have: Ronaldinho, Deco, Thuram, Zambrota or Edmilson.)
The season before Guardiola came:
1 Real Madrid (C) 85 points.
2 Villarreal 77 points.
3 Barcelona 67 points.
The season after:
1 Barcelona (C) 87 points.
2 Real Madrid 78 points.
They won the treble that year.
Rijkaard was far from clueless, but Guardiola took Barca to a whole new level. (And naturally I'm biased enough to think Mourinho did something very special by getting the league's record number of points in Guardiola's last season. He went up against a team people say was the best ever and he got the record number of points in yet another major league. (Whenever he leaves a country, he is the holder of the record number of points.)
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I think the value of having one of the best managers ever is huge. I'm more scared of Chelsea with Mourinho than I would be if they had Avram Grant but signed Messi. United without SAF is a bigger example of weakening than when they sold Ronaldo.
Look at Liverpool, wasted about 100m and went backwards. QPR, wasted a ton of money and also went backwards. SAF's managerial style was to be director of football and head coach, he would have done far better in both scenarios.
It's impossible to say SAF = +X points per season though... Each season it differs even at United, so it's impossible to say if he had been given Jose's Chelsea job what would have happened.
I think Mourinho will retire as the greatest of all time though. He constantly goes up against the best managers, the best teams, etc... If he spends a few years in Germany and France, if he wins a few more CLs, if he keeps breaking records, there will be nothing left for him to do. I personally think the record he had about home games was the most impressive... It's really scary to be able to put your house on Mourinho not losing at home, if he does end up at Chelsea next season and goes unbeaten at home in domestic competitions again, I don't see either of the Manchester Clubs beating him to the title, they both tend to rely on home form.