You wrongly assume that extending Yaya's contract is proof that we aren't going to sign a new CM this summer. Right now Yaya's fitness levels puts that of some of his contemporaries to shame especially if you factor in his age. Would you rather keep Delph and/or Fernado?
We looked worse without him in the side in the second half of the season so the problems are clearly not 'down to him' like you suggest. Going into a new season, the prudent thing to do is sign a young and talented CDM and thankfully the two events of keeping Yaya and signing a CM aren't mutually exclusive.
I'm sure the poster was simply implying that he currently is the better of the lot compared to what we have, injuries and other factors considered however going forward he can more than do a bit part job.