Mou will move back to RM, don't worry.
Its speculated that Perez wants Mou as the next manager and Xabi Alonso as his assistant. The only way Solari will stay is if he wins the CL.
IDK why Mou is always in demand, he hasn't done anything notable(something that makes elite clubs go for him like crazy) since 2010. His teams are boring to watch most of the time, he looses most of the dressing room, and add to that his preference for defensive football at all costs, which usually don't work out. This has happened at RM, Chelsea and UTD.