yeah but sahin isn't a DM, he's a deep lying playmaker just like jack. he's not physical and he can't tackle. if we are seriously relying on sahin to take song's role then we're in serious danger, mainly because our midfield would be depleted of any physical presence. we struggle enough with song there but if he goes and we put sahin in his place we'd get overpowered every game.
you could put diaby there but he's crocked all the time so once again, we'd be in serious danger 2 months into the season when injuries kick in. plus i think diaby plays best in an attacking role so it would be a waste of talent anyway.