M'vila is proven quality though, which Pong and Le Coq (heh) certainly aren't. Yann is only 1 year older than Coquelin, yet is already a leader for Rennes and the defensive shield for the national team - something worth considering. Besides, I think Coquelin would be better suited to play the B2B role next to the DM, and Frimpong frankly hasn't done enough to be the immediate backup. He's also always injured anyways.
Besides, isn't Song leaving next year to the ACN? And in any case, he has hardly played as a strict DM the way Yann would. Then again, we might be getting Vertonghen which would make the added signing of M'vila overkill.
I think M'Vila is a quality player who is destined to be one of the best defensive midfielders in the game, much like how I saw Mascherano all those years back in the Confederations Cup (before he signed for West Ham) and knew that he would be great.
The only really question mark over this signing in light of the news that we've offered Song a new deal is that IF we were to sign Yann, where would that leave Alex Song? As much as I appreciate his assists this season, I don't think his link up play or passing game are good enough to warrant being moved higher up the pitch.
