They're not mental Mark. If Alisson is still performing and justifies giving a new deal then he'll be given one, just as Salah was.
If the club believe that this Valencia keeper is a bit of a bargain, as has been reported, then it's a no brainer. The reports coming out of Spain suggest he's happy to be loaned out for 2 years - a lot can happen in 2 years. Alisson's form or fitness could be an issue, he might decide he wants to move to Spain or Saudi. Who knows. In the even that in 1-2 years time we need a new keeper, the club obviously believe this guy is best placed to replace Alisson. And if we don't need to replace Alisson, we can always move this other guy on.
It has no impact on our ability to make any other signings so I see no reason why they shouldn't do this deal if they believe this guy has the potential to replace Alisson if and when he needs replacing?