I think Milner's been perfectly fine. Good, even.
I personally can't remember (and it may well be because my memory's failing me) his defensive errors costing us in 2017.
It's not his defending that's an issue for us, and even going forward, he offers more on the ball than most fullbacks.
The trouble with him is, he mostly cuts in onto his right, and that's a massive issue for us when Coutinho does too. We're have little enough space without both of our left-sided players cutting in.
I'd go so far as to consider playing him on the right, except he prefers the left and then we'd be left with Moreno on the left.
It's just one of those funny situations where the player is not really at fault, but he's not what the team needs.
It'll be interesting to see his role in the team next season, if we get a proper left-footed attacking fullback. I'd miss his professionalism and penalties.