Weirdly I still think it’s below his value. If he plays well for us this season at any point we could definitely command more. He was the star player in the England u-21s and we’ve seen the likes of Maddison go for 20 Mill. What if he turns into another Sterling or a player like Malcolm or Dembele? His value in the market could be huge.
I get what you are saying but there is a lot of conjecture in your post. The only way gets in our side imo is either through squad rotation (which won't happen often as we're not in Europe) or if our first choice attackers go through crappy spells or get injured which I hope doesn't happen. If he isn't involved much this season his value definitely won't stay at £28mil unless he goes out on loan again. And if he isn't involved much, I can only see his desire to leave increasing further.
Like you, I have a lot of faith in his potential but looking at the wider football landscape, I don't believe there is any kind of logic you can use to suggest that he is currently worth 10mil more than Maddison, who you use as a comparison. Leicester didn't pay £20mil for Maddison because of what he had done in England youth teams, they payed it because he had a stunning season in the championship in a terrible team. Lookman had a very good 4-5months in Germany in a very good side so I'd say their values are about the same.
I was hoping he was going to be our first choice left sided winger this season, but with Richarlison's signing and subsequent impact, I don't see a place for him in our first 11 and I bear no grudge toward him if he wants to leave for a team where he is very highly thought of and the prospects of first team football are much better.
It would also leave us with a decent wedge to go into the January window with if we need mid season cover.