Whilst I don't rate Sanchez as a top defender, I don't think he's as bad as some people believe.
Over the last 2 seasons, how often have we actually had a settled back 4? Between Alderweireld, Rodon, Dier and Sanchez there has never been consistency in a starting pair. Add to that the 2 full backs changing quite often.
If the right offer, like the swap with Sevilla, comes in then yes, sell him. But I'm happy to keep him for early cup and European games.
However, it is paramount we sign another centre back, and if that means selling Sanchez or Dier, then do it.