The entire point is that we don't forget about the injury. Why are you having such trouble grasping this?
It was a yellow card, then upped to a red when the referee saw the injury. That's the rule, as many other people have already pointed out. If your foul causes a serious injury to an opponent, it's a red card.
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Source? It is the offense and not the outcome that is decisive.
"A tackle or challenge that endangers the safety of an opponent or uses excessive force or brutality must be sanctioned as serious foul play.
Any player who lunges at an opponent in challenging for the ball from the front, from the side or from behind using one or both legs, with excessive force or endangers the safety of an opponent is guilty of serious foul play".