Sell. We have a one year optional extension, but if we keep him next year,
a) even if he regains some form, he will not justify his current wages. Thus, next summer he will have one year left and high demands. No club will pay a fee when they know they can get him for free 12 months later, or no club will pay the fee we would like. We will be again in a Lingard situation.
B) most probably he will not regain his form, so he again no club would spend alot for him.
So no matter what his value is now, in both situations next year his value will be far lower.