I just think he's very young and very inexperienced (20 senior games?).
Standard isn't great in the A League but back there he'll learn how to be the main man in the team, how to deal with the pressure of that.
He was fantastic last year, but if he is to become a top player he'll need to perform with consistency despite being a marked man. I don't think he'll get that at Celtic where he'll be one of a number of top players playing under little pressure most weeks.
If nothing else, a year of A League will toughen him up, and that will be good for him as well. When it comes down to it, it's not his technical ability that will need to improve for him to make it with us, it's his strength, decision making (already very good for his age) and the mental side that he needs to work on.
Why is so important that he learns to be the main man in a team though? He doesn't need to be the main man.