If he stays...
Assuming he is money orientated and that he will be playing for a massive signing-on fee somewhere, we should get one final Flamini-esque year from him.
The question is, will that one year be enough to offset the loss of revenue his non-sale represents and garner the money needed to replace him? On his own, he isn't enough to make the difference between winning the EPL or not. But is he the difference between 4th and 5th? That in itself would equate to £25m of lost CL revenue.
If we sold him now and used the cash to buy someone almost as good...
Most likely puts the 4th/5th further at risk but means no additional expenditure in replacing him.
Then again, maybe that is too literal. We already have Gervinho, maybe the money would be better served buying a centreback.
Overall, I said sell. I just hope holding onto him doesn't turn out to be a strategic error.