Good move from Brendad trying to motivate him. Lets see what he can do My opinion is he will end up a decent midfielder but perhaps not at the top level. He does have great moments though on ocasions which makes me think he could be better than he has been.
He could be a useful player in the future, not a world beater, but useful. At the moment he has too much to do to improve. Needs to work on his fitness, positioning and temperament.
I don't see Shelvey making it. Too many flaws to his game and he doesn't strike me as being the sharpest tool in the box either. I think he'll be a very good mid table midfielder, but I don't ever see him squaring up to the quality of the top 4 or European teams (unless it's to start a fight). Even as a backup squad member, I don't trust him to stay on the pitch or to consistently come on and make an impact.