To my eye...the issue starts when Milenkovic is on the ball. Marmoush and Gallagher are both there. Marmoush should press him. Tonali watches Delap run past him showing for the ball. Why does he not go with Delap (I believe). What is a he guarding there? Gibbs-White? Why hasn't Robertson tucked in, who is HE marking/watching? As for Tonali, he let Delap run and receive that short pass unchallenged (and effectively allowed him to play that short pass to McAtee who plays it past Tonali anyway - he's slow to see that ball).
Of the two CBs, the one I'd criticize there is JPVH who has allowed MGW to remain onside; if he pushes half a yard MGW is offside.
I think RDZ wants us to play higher when possible. What I cannot figure out is why neither Marmoush or Gallagher pressed their initial men, and why Tonali didn't either follow Delap when he showed for that ball from Milenkovic, or then read the ball from McAttee better/not notice MGW running in behind him?
I can see why on first glance it looks like a CB positional error, but a closer inspection (IMO) shows it is a systemic breakdown which for me is an example of a side which has not played much football together and is still finding its rhythm.
FWIW...I'm well up for exchanges of this nature, and discussing/debating points (I learn too). I'm personally not up for repetitive 'he's not a good defender' when it's clear he actually is, plus it's clear it's an opinion you hold which we'll never agree on.