O'Donnell has played at left back a number of times and is a natural full back. If you are making someone switch it's far better to switch someone like that than stick Grimmy there. Positioning is half the battle and if you normally play full back it comes naturally.
I have zero confidence in Lamie at left back after the Livi game. He was a mile out for most of it, 100% to blame for the 1st goal and played a big part in losing the second. Any time Livi attacked down the right wing he was caught out. (I don't blame him for not being very good, as it's not really his position).
The irony is we had Carroll, Dunne, McGinley and even Donnelly that can play left back and only Grimmy on the right - and we are struggling to fill the LB slot.
I'd be delighted if Lamie's performance was just a temporary blip and he plays a blinder there in the future with O'Donnell at RB, but I'll need convincing.