Chill out mate, : ) It's your right to disagree, but turn it down a bit. Thanks.

I'm not level 7 yet (and perhaps, I will never get there, I might stop before...), so I can't speak for those levels. What I explained has worked for me during the past 2 seasons that I've been playing, and I don't see why it shouldn't work in most cases.

My own problem was to keep "bad" players in my squad, which made me at first very vulnerable, when I got multiple injuries. Now that all 22 players are on equal footing, I guarantee you this problem is gone.

Yes, sometimes when you get really unlucky, you might have 4, 5 of your 7, 8 defenders out. There's the time to use those red boosters. But that scenario is much more rare than what you're suggesting.
Changing formation shouldn't be an issue, neither, there's always more than enough formations to do well against any other one.

Also, a mistake I used to make is train players when I already had 3, 4 injuries in the squad -- you shouldn't do that. Past 3 injuries, you should stop the training a while to prevent having even more.

You may do as you want with my point of view, and experience, : ) all I can say is, it does work for me, and I'm happy with it.