Melo is naturally gifted, you can see that from his ball control and great touches. All Delneri had to do is to teach him how to use his talent and concentrate.
Sissoko, on the other hand, is not a football player. At least, not a big team or even mid-table team regular. He can't control, can't pass, can't shoot properly, stupid tackles etc... The only thing he used to be good at was his incredible stamina and drive which are lost now. Nothing can be done! SELL SELL SELL