Let's not go overboard on Jason as a prospect. I'd hate to give him up right now but while he's flashed a bit in his loans, last season was not quite the loan that Celtic was. They seemed to like him at Gala but he was played out of position most of the time. There are obvious red flags, as well. He's not the biggest player for a CB. There are certainly a bunch of CBs who have been a bit on the small side who have made it -- it doesn't preclude him from being a great player, but it certainly makes the task a bit harder for him. I guess I feel like I could see the attributes that might make him a star but I can also see the attributes that might make him never amount to much more.
Remember, we had everyone here typing many of the same things about Rekik (another smallish defender who was knocking on the door of his NT). And while I'm not really willing to write either of them off just yet the path of a high caliber CB is a long one that often has a few twists and turns and a lot of learning along the way. We shouldn't overrate Denayer, there's a very obvious path where the physical side of the game is too much for him in the PL and he simply can't play at the standard we require. His ball skills, however, seem above average and I'd be really surprised if Pep isn't even interested in taking a look to see whether he can adapt at the level required.
If he were to go to Arsenal, I'd be extra surprised if it were part ex for Bellerin. I also don't think he'd play week in week out for them, either. Bellerin seems like one of those juicy rumors City might even make up if Arsenal start tapping up Denayer in order to remind the other team that two can play that game. If we were to sell, 15m seems too low to me. He's shown enough promise that I'd probably want 25-30m in this market but might settle for 20m. He's less proven than other younger mostly potential defenders but not so much so that I'd let him go cheap.