Basically unless he has David Silva's ability and awareness (which he doesn't) he needs some physical strength/stamina to match his talent. Tadic and Mane were brought in by Koeman at Southampton who have creativity and in Tadic's case a big eye for a pass so I don't buy the notion that Koeman goes for just workhorses.
Ironically enough him and Adama Traore came through Barca's ranks together and a fusion of both would be a great player. Traore has the tenacity and energy that Deulofeu needs, and Deulofeu has the crossing and end product that Traore needs.
Sell him for £10 million and let Walsh bring in a good replacement. Mahrez or Perisic are dream signings, although I think we'll stick to 3-5-2 next season and go for Rooney up top with Rom and not play wingers.