Makalele trains Diarra personally. He is being groomed to take over his spot once he retires. I doubt Chelsea would let him go.
I dont really c the point of this buy other than squad depth. Diarra isn't a star yet, and is in the same boat as Paro in terms of potential to be big. Juve dont need him, but getting him wouldn't be a bad thing. Our defence is where the attention needs to be focused.