I'm not sure McCarthy playing on the right of the three would fit into Jim's system. Think of how Tierney plays in the Scotland back three - advancing up the field, swapping with Roberston, basically an attacking defensive player. I think he's trying to get Fraser to play that role, and as a full-back, the attacking part comes pretty naturally to him. The perfect example is the win at Parkhead last season. Fraser was pushing up all the time and gave Celtic a real headache. I don't think McCarthy would fit in that role.
Edited to add: I think McCarthy is excellent. But Shaughnessy is captain and performs a vital role for us. And now Dunne has given us some left-sided balance in the back line. Normally I'd have McCarthy as the first name on the teamsheet.