I think Dack would be hard to replace. I don't mean we couldn't get an equally good player; it's just that his specific role between the midfield and the attack has been at the core of the team's formation, so we would probably have to re-shape if we couldn't get like-for-like. Mowbray seems to me quite capable of doing that, because I suspect he re-thought his whole tactical plan for the season after those first two games. For example, did he or anyone else envisage Smallwood being at the heart of the team in every game?
All the same, I suspect TM will be fighting to retain Dack. More likely, I think, is cover for Graham.
I know Saxo would favour a proper target-man, and I agree that there's something to be said for that. After all, a lot of Dack's goals have come from his ability to feed on the loose ball. (I thought Džeko looked decent for Roma. Perhaps we could get him. )
As Champs says, though, if someone comes in with a massive offer, who knows? I suppose a lot depends on how much extra Venky's are willing to fork out.
As I said on the relevant thread ten days ago, I'd rather see us sign two or three good players than half a dozen of about the same standard as we have now.