The left*Arteta is trying to be too clever playing Tomiyasu on the right instead of being pragmatic, he should go ask Pep how that works in the CL. Tomiyasu offers nothing going forward on the right which makes Martinelli ineffective as well.
I agree though. It was a "tactical" decision against Salah, but now it feels like something else. If Tierney isn't doing enough to start at LB then tell him that. Don't tell a player its a "tactical" decision and there's nothing he did wrong.