I would consider myself as one of a few on this site with an informed opinion as I have seen him play around 50 times for the reserves.
I like Tierney. I think he is a good full back but a bit too small for CB. As a full back his defending is good, he is quick and gets forward well and puts in a good cross.
I think he could make it, but it depends on whether he gets the chance and whether he wants to stick around to get the chance.
However, I followed his progress at Crewe last season and towards the end of the season he was on the bench a lot, which is not a good sign for his progress.
However, I believe if he does not make it at United, he will still have a good career in football. Could end up like Danny Higginbottom.