If Liverpool really is serious about signing Alexander Isak then it would be best for them to offload Nunez first because I don't think Nunez has much to contribute, even though he was signed for a very expensive fee. So if Liverpool can sell him, Liverpool can at least minimize their losses and can use the funds to bring in Alexander Isak.
Alexander Isak has had a good run at Newcastle, so it stands to reason why Liverpool is interested in him and it looks like Isak is keen on Liverpool too. Now we just have to wait and see whether all of this will come true, and if Newcastle set a reasonable price for Alexander Isak, then the transfer looks likely to happen.
Nunez could still remain in the club and serve as a backup or an attacking partner with Isak. But Liverpool might want to sell Nunez to balance their accounts. With the performance of the Uruguayan footballer decreasing, Liverpool might not be able to sell him for a high amount. Clubs will offer very low price for him, which means Livepool will seek for funds to bring the Newcastle United goal scoring machine. Liverpool will have lots of transfer issues to settle with some important players not extending contracts.