He just turned twenty. He's been at four different clubs already, played for the first team, on loan, in the reserves, in Europe. I think he's been bought with the idea of being the long-term replacement for Gerrard, and Gerrard has got a few years in him yet. Shelvey isn't a defensive midfielder, and isn't a wide player. So, he would have to replace Adam, Gerrard, or Henderson. But we have been trying to win every game, desperate to get into fourth, or win a cup or the derby and not scoring enough goals. An attacking substitution is going to be Bellamy, Downing, or Kuyt before Shelvey.
He's very young, has a lot of varied experience for that age, and is behind better/more experienced players when results really matter. I would expect to see more of him and Coates in the League from now on.