If the intention was to use Niall McGinn in a defensive role, stopping Tierney, we'd have been better playing a defensive midfielder on the right hand side - McGinn is a winger who gets to the byline and puts crosses into the box - he's not a defensive player. Would've been better sticking him up front through the middle where he could've caused Celtic more problems than May, and sticking another defender man marking Tierney if that was the plan. Any time McGinn got the ball he was in his own half with no space to run.
The guy must be pretty annoyed that having made a blistering start to his return to AFC (must've been in running for player of the month with some of his earlier performances) he then gets stuck in ridiculous positions against both Hibs and Celtic. Probably ruined his confidence now so that when he does go back to his normal position he'll not be firing in all cylinders.
He might have helped nultify Tierney a little bit, but a 2-0 defeat is the end result, so I'd rather have seen McGinn doing what he's good at doing - ie attacking. If McInnes wants to set up uber-defensive, don't bother picking McGinn, it's an insult to the guy and doing him a dis-service playing him to man mark a left back.