I can't see Mary changing a winning side, but in my opinion sooner rather than later he should start Lomax for a few reasons.
Obviously goal kicking is the big one.
I do think Aitken has started the season well but in my opinion his game has 2 glaring issues. Number 1, his passing game is non existent, and when he does attempt to pass it's so bad it's comical, he looks so uncomfortable passing, like he's never seen it done before. Number 2, not his tackling, but his defensive decisions, he can tackle fine, but he frequently is out of position to make the tackle, he's jamming up too fast, or hanging back too long. Lafai has been excellent the last few weeks getting this timing right and it's critical.
Now for Lomax, he is absolutely wasted sitting on the bench, he's there until either someone gets injured, or the game is over one way or the other. I would start Lomax and move Aitken to the bench. If an injury occurs anywhere in the backs, Aitken comes on into the centres and the back line reshuffles. If however, an injury does not occur, you can still use Aitken in the middle to cover 13 and give Lawrie a rest.
Doing this, Lomax is on the field for 80 mins and you have a gun goal kicker, we are not wasting a bench spot having a guy sit there for 80 mins just in case. Aitken can play in the middle, he can run hard, does not have to worry about passing and can defend in the middle where he does not have to make those defensive decisions on when to jam in, he can just get in and tackle.
I think this is best for Lomax's development, but more importantly, best for the team.