I disagree to a point. Sure, he is our starter and we want him starting as much as possible, but results so far this season are better than expected when Hakan can't play or is starting on the bench. It's basically the same, about 2.1-2.3 points gathered per match. I do agree that we play uglier without him, but there are instances of matches we won by scoring 2, 3, 4 goals when Hakan wasn't playing, so it is doable. There are also games where him or Mkhitaryan don't look good at all and yet we win. Crazy stuff.
I don't think we can find a player like him for cheap to replace him adequatly (that meaning, somebody on the same level), but oddly enough, it's him who can be replaced easiest if sold. Maybe Pavard as well, since we have Bisseck waiting. Even if that means one year of Zielinski and Asllani playing in his position, it's not that bad at all. The only player that I really like to be his successor is Ricci.
Of our main starters who aren't too old to be sold for a good fee - Barella, Hakan, Bastoni, Lautaro, Thuram, Pavard - it's only Hakan and Pavard I believe could be adequatly replaced and could make sense to be sold (even tho I hope we don't have to do that). For Lautaro, we would lose our talisman, striker captain, Thuram has been arguably our best player this season, at least in the first few months, Barella is our best player overall, Bastoni is something special and CBs that can create so much are not everywhere. Especially those who are lefties. It's only those two, and Frattesi on the bench who could be sold and I wouldn't worry too much.