With strikers, the first thing that goes is their pace, the last thing to go is the finishing, movement and ability to find players. Ole lost pace after his injuries, but his finishing and ability to get into goal scoring positions was always there. That's why even after a long layoff he always looked odds on to score if he got into a good position. Injuries rob you of a lot of natural ability, but if your game is based on intelligence and clinical finishing then you're probably going to be ok.
This is also why I think Ruud was able to come back and will still score goals for Hamburg. His pace is gone, but the mans natural goalscoring instinct and ability to slot away chances will remain with him till he decides to quit the game.
That's where penalty box predetors and natural goal scorers can add an extra few years to their careers. But defensive midfielders aren't so lucky. Hargreaves game while definitely involving skill and positioning is also about stamina, power, pace and tenacity. I'll be shocked if he still has the same pace. It will probably take him the rest of the season to build up to the same type of stamina, and with regards to his power and tenacity, well some of that is a mental issue. A player coming back from serious injury might think twice about going into a full blooded tackle. If Hargreaves turns out to be that sort of player I wonder just how effective he will be.
Just look at Cech after his head injury...he looks a different sort of player. Sure he'll stil have games where he shows some of his old form, but the injury has changed him. It's made him less brave then he once was. The armor has been cracked and for some players once they get seriously hurt they begin to doubt if they are ever going to be the same again. I have a feeling Hargreaves comeback will test his character and metal makeup, and from what i've read there seem to be doubts in his mind if he's ready. I think the fact he wants to take it slow will most probably mean we will see the return of a very different sort of player. (hope i'm wrong, but that's just what my gut tells me).