Here is Celta schedule for the next two weeks...
So they'll have a week between Manchester and RM games. I won't have my hopes up because RM will have a lot more to play in this game than Celta but based on Berizzo's rotation strategy so far I think it's quite likely they'll at least play with their best XI against RM. Europa League final is on 24th so they would again have a full week to prepare for if they can go past United. And if they do that then they could use RM match as some kind of preparation for EL final too.
Good observation about rotations. Seems logical they could rest vs. Alaves.