I remember when my son broke his leg as a late teen. He broke it just below the growth plate so the healing would be a bit longer and he would need a full length leg cast. When they took his cast off he had lost possibly half the size in his calf and thigh. Just a few months of non-activity for athletes can cause significant atrophy.
And importantly, the atrophy isn't necessarily going to be just in the area that's injured.
Crazy to think, but a person's entire body is connected by various systems, and an injury to one part can have lots of subtle and not-so-subtle effects on other parts of the body.
Anatomy! Super fun.
And that's not even touching on the fact that a super traumatic physical injury very frequently comes with a whole suite of psychological complications, from actual PTSD to depression.
The people who dismiss Hayward's injury and ridicule him for not being all the way back yet are, in a word, ignorant. Either because they're not aware that recovery from major injuries is complicated, or because they choose to be ignorant.