The Pogba reference is not quite fitting though. Pogba didn't get game time at all because Fergie was busy getting 60 year old Scholes back from retirement. Thiago has had over 100 caps in 2 seasons, despite suffering a ligament injury, in a squad with Xavi, Iniesta and Fabregas in midfield. Now he believes he has better chances to make it to Brazil by playing more somewhere else but he's not going to get called up for the World Cup next year by playing for Manchester United. That's a guarantee. Spain's system = Barca's system. If he can't compete at Barca and decides to run away why would del Bosque nominate him for Spain?
Sorry, but its not a 'guarantee', or even close. If he did join us, United do well in the CL with and he has a stand out season, then he'd have a good chance of being called up I'd say.
You're right about the differences with the Pogba/Thiago comparison, but there are similarities too. They are both young, talented players who want to play more, and the fan reaction is quite similar. The point was just simply, that as a United fan I can empathise with your frustrations if he does leave.