I'm not sure we should sell him. Hear me out first before deciding ive lost my mind.
As I see it he will be third choice if he stayed for next season.
Now heres the thing, if De Gea is our number one I can see Henderson leave next summer unless he gets assurances about him being number1 next year and De Gea won't stick around being a back up. If Henderson gets the nod this season I can see De Gea moving either in Jan or Sunmer.
Either way I can't see De Gea and Henderson (beyond this season)being a back up. Which means as soon as one leaves we are going to need back up keeper.
Maybe I'm missing a youngest at the club that would take the back up slot? If there's no one it might be worth trying to keep Sergio. Having said that he may not want to not play all season only to end up and at best a back up again.