I've been thinking the exact same thing.
I knew he'd get criticism, because of the amount of space which Gunter kept on getting over on their right-side. Amazingly though, Gunter isn't a right-winger - he's a right-back. So not Fox's man to mark. Stopping him would be the job of our left-midfielder. Now who is that again?......
Don't get me wrong, I'm not having a go at Lallana here. I accept that his defensive game is poor and he doesn't help out his left-back as much as other wide players (as much as I dislike Harding, I have ALWAYS said that the one thing in Harding's defence is that he isn't helped by Lallana). So if other people are with me on this, and also refuse to criticise Lallana, then they cannot criticise Fox. Majewski, who seemed to be their midfielder furthest to their right, was constantly coming in field - so of course Fox was narrow. We'd have been over-run in the centre of defence if Fox had stayed out wide.
If anything Fox should be getting credit, because time and time again he blocked or cut-out Gunter's crosses (effectively saving Lallana's skin). I was very impressed with that.
I will need to have a closer look at the highlights to see if he was at all at fault for their first goal though. I was too busy watching the ball to see who was/wasnt marking Derbyshire and/or keeping him onside.