It's a terrible deal for us. Where's the benefit? He's had 18 months there, he's clearly not shown to Pep that he's good enough at this moment and i'd question how much better he can get playing in that standard of league when it's just going to be more of the same for him. To constantly improve at anything in life you need to be challenging yourself, i'll say it now, he'll stagnate at Celtic where everything is easy for him. Should have had the balls to test himself at Southampton and we should have insisted on it, or at the very least denied him going back to Celtic on loan.
Also don't buy into the whole "he'll play in the champions league" thing. For a young lad, sure, playing in the champions league is a big and nice thing for him, but what is likely to be 6 games maximum in a season isn't enough to make you better, especially not compared to an entire season at Southampton against quality opposition every single week. It shows a weak mentality from him and unless that changes, he'll never make it to the level his potential suggests.