Sainz and Hamilton will give a hard time for the team but I guess in this case they can have penalties on their salary and they can be benched for the 3rd driver.
This will only be bad if they take it personally, but the Mercedes team and Lewis have definitely a professional relationship and I don't think they will have a hard time. It's just that their contract and loyalty to Lewis weren't any more up to par with Mercedes that's why he wanted to be in a team that actually wants him, and yearly contracts for Lewis aren't gonna cut it. I think I understand why Lewis decided that.