Why bother with Fabregas when you've got Thiago in your ranks? He'll turn out better than Fabregas.
Different players. Thiago is more of an AMC, Fábregas would fill in for Xavi.
Also, despite his undoubted quality, I'm afraid Thiago won't make it with us. He strikes me as very impatient and will want a move if he isn't starting regularly at the end of next season, which won't happen.