This means there are two things that we are not sure about. First of all, we do not know who will get Sainz, that part is still unknown, but we also know that in 2026 Audi will get him, we just don't know who will get him for 2025 just for one season.
Secondly, we do not know who Mercedes will get instead of Hamilton. Anyone has any clues about these two?
The most discussed option for Sainz is Mercedes now to tell the truth. As Hamilton is leaving for Ferrari his seat is going to be available. Mercedes has Mick Schumacher as their reserve driver as well but would you expect him to get the seat? I don't think so.
He still couldn't prove himself to be reliable to drive for a big team.
Especially while Sainz is doing wonders at Ferrari this season I believe he would get more attention by Mercedes. Probably there won't be any space opening at Red Bull or McLaren next season so Mercedes would be the best option I think.