Thiago was a risk for a foreign coach. He stagnated in his team - it was not only the lack of match time - he did not even play very well in this time last season. Too much risk to put in the talent that he was without knowledge of the league - maybe with a question to how he would perform in the EPL.
I can fully understand Moyes with this. Maybe some homegrown players get time - but not foreign talents that cost a big transfer sum...
I don't know how he was a risk. His version of stagnation was that he couldn't get into the team in front of Xavi/Iniesta/Fabregas and wasn't favoured by Tito who funneled the entire team play through Messi. The year prior under Pep, he got significant minutes, which considering his age and the people he was fighting within the squad to get a place from shows how highly regarded he was. Not to mention the fact that Pep wanted him at Bayern, despite their CM riches already. Even "stagnating" as you put it, he would still destroy Cleverley/Fletcher in their current form and would have been a great partner for Carrick. Also, the fact that he's picked for Spain and Mata can't even get into the squad says something.
And I don't know how you can classify his transfer as a "big sum". He release clause was a ridiculously low 25 million Euro, which considering how dire our options are in CM, and how in need of a technical, progressive player there was peanuts. We paid 27 million pound sterling for Fellaini who is a decent player but doesn't have the ceiling Thiago has. We payed virtually that much for Young and Valencia each!