Everything you say there is valid and difficult to argue with, although I'm giving it a very good try

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I still maintain the difference between the league and cup games is more nuanced than just the 1st choice back 5 being fit irrespective of what stats say. Tbh we don't even know his first choice back 5 apart from the GK and CBs I guess.
For me, there is a clear difference in how we set up in the cups as opposed to the PL. We are much more pragmatic in the former. I think this partly supports the argument that we have a decent squad but it's the system we play that is the reason for our poor league results. But equally could support your argument that we can play his preferred system if we have our first choice back 5.
I personally lean towards adapting style of play as injuries are going to happen and your first choice team is rarely going to be available. Good discussion though mate.