Every team above them have scored more goals (and quite a few below them as well), so this isn't just a blip season or a few dodgy results that have affected their goal difference - they're just not scoring at the rate of previous seasons. A GD of +6 after 11 games is embarrassing after what they've spent. Ok, so ours might only be +9, but 7 of the 17 we've conceded came in one game.
After match week 11 last season they'd scored 34 goals already and were on +24. At the same time in 2018/19 they were on +29, and in 2017/18 it was a whopping +38.
This season they've scored 17, so their goals per game average thus far has halved compared to the last 3 seasons. It's not just Aguero being out - their chance creation and conversion has fallen off a cliff.