Don’t sell him. No matter the price. For starters, I don’t back Unai to find a replacement. His hit rate so far is one player. This one.
I think he probably should be sold to rebuild the squad, but you have hit on the problem and the reason I do not want a rebuild. Emery is personnel man is sub-standard and, because as manager he needs to win now, he buys players that will do this without looking to the future nearly enough. It is a difficult balancing act, but the purchases/loan-to-purchase of Guessand, Elliot, Diaby, etc. suggest that his eye for players that suit his system is not as keen as one might like. I love Unai, mind, but it is a concern.