Played my first EFL game after 5 friendlies and already my squad has 6 injured players. I seem to recall this was an issue in the early days of FM18 too.
a good medical staff and infrastructure can help.
I've face the same issues and a good preseason physical preparation with a good medical staff have helped me. the last thing, during the preseason cancel all the individual training to keep your player focused on their stamina.
Good game time