Did you read the note.If not then.
A player in real life get injured,so he is treated in hospital,but in this game player can be imediately treated by red packs.
The motive of lossing energy while injury is that he is not fit for playing matches at that moment but it can be resloved by few green packs.
If player is hospitalised,then player need rest and time,as player is given some free time to rest so some condition is lost.
Its very tough matter you may not understand.