If the bid is true and Juve are a bit desperate, it might be possible to milk the fee a bit more to say 25 mil. Then it becomes a bit of a puzzle of what to do. Firmino has had a lot less input than Mané, especially recently, then in addition he's a year older. It might reach the point where the offer is too good to turn down. On the other hand finding an able replacement in the right age range and determining whether that player fits in with Jota's versatility is difficult.
It looks like Belotti is still available and he might be a willing backup #9 and may well be an upgrade on Origi as a supersub. However, I think the wages might be a bit too much for a pure backup there even on a free.
Núñez is likely to be the number one striker and then Jota basically is the first backup across the full front three. So where would Firmino's replacement actually get to play? Is Carvalho good enough to not warrant Jota on the left side? Quite a difficult set of circumstances should Juve be willing to break the bank for him. Obviously not an ideal time to lose a very good squad player, but at the same time every player has a price.