First of all, I rate Zinchenko. Everyone does. He's the starter at LB. But I get the sense that Arteta plays favorites with him, and the flip side is that he doesn't trust Tierney. Yet, he has no qualms with throwing on Kiwior at Anfield. It doesn't really compute.
I still rate Tierney as one of the better LBs in the league, and he offers athleticism, pace, and defending that Zinchenko doesn't. He's markedly better than Z in these respects, and offers tactical flexibility. If we've got a lead in the 60th minute, to me, it's almost an automatic substitution (which is what often happened at City). Yes, it does possibly involve a change in system, but we should be training for this. And depending on the matchup, Tierney should be getting spot starts as well.
As it is, I think Arteta got it wrong in the 2nd half on Sunday and it seems we're likely going to lose Tierney in the summer. I'd rather a player who can genuinely offer something different rather than a backup who's just a younger and worse version of Zinchenko. Then again, maybe our medical team has determined that we can't rely on Tierney - if that's the case, then so be it. And if Tomiyasu is physically ok, then he provides the defensive cover that can invert, which is what Arteta wants. But as much as I loved Tommy last season, he's had a horrific season this year (even besides the injuries).
