You are right, but there's definitely a correlation between form and injuries, particularly when it's injuries to key players.
You only really need to look at the teams at the top to see this. Liverpool haven't really suffered them this season so far. Yet go back to when the lost VVD and you'll see their form dropped off.
Woolwich when they lost Saliba, and now when they had Odegaard out, then Rice and now Saka.
City you've already mentioned.
Newcastle last season.
With us, losing VDV, Romero for long spells, along with Bentancur and Vicario, not to mention Richarlison who, pre injury, was on a Purple patch, it adds up.
Injuries to one or two key players can have a big impact, as the above examples show. Losing four or five will have a greater impact, as Newcastle clearly showed last season, and as we've shown both last season and this. You also need to factor in the knock on effect it has on the rest of the team. We've had multiple posters here going on about how our players look knackered, well when you are missing half of your squad, is it any wonder?
Let's also not forget that any long term injured player doesn't just magically return in top form. They're not match fit, they'll have lost that match sharpness. That takes time to get back, and it's exacerbated by them playing against others who are both fit and sharp.
I'm really not sure how this can be overlooked or cited as "just part of the game."