I think you're being a little harsh on Guendouzi, Barnesy, and I agree with HP that Arsenal will ruin him. He's playing in a midfield which is being run by Granit Xhaka. That would affect most players, especially a young player still developing.
Don't really want to see Milner starting any future games in midfield (maybe only in the domestic cups), but right now if we need to give Robbo a rest, Milner's playing leftback. This is a worry.