Drogba is the leading talisman for Chelsea. Where would they be without him? Can Grant change a team's playing style mid-season after they have played that way for 3 seasons? It probably isn't impossible, but it's highly difficult. Hiddink did it, but how many can be on the same level as him.
Chelsea bought SWP for 20million, and Kalou has been better than him. Kalou creates chances and scores goals. SWP runs and runs some more. He's probably got the talent, but he's just another overrated English player. I wonder which one of the United players you're thinking of, because I honestly can't think of anyone who can be worse than SWP who plays in a similar position and has played more often. If you're referring to Park, then you're wrong, because Park took us to Moscow. If you're referring to Nani, then you're also wrong, because Nani won us games. If you're referring to Ronaldo, then you must be Ian Wright.
Like I said, it's likely that he'll be sold anyway. Losing Robben to Madrid probably cost Chelsea quite a few goals throughout the season too.