Often times in media, medieval battles will depict a scene where two knights supported by their various squires will enter a melee/brawl after the formations and charges are broken. Knights are typically portrayed as wheeling around one another trying to unhorse or simply kill their opponent. In reality, if this duel was interrupted by a third soldier killing my horse, would my training allow me to unhorse myself in a way that reduces injury? Or is it more likely that my horse would fall to the side, and my leg would get crushed, most likely killing me? What are the quick, life-or-death options for a knight whose horse has been killed beneath him? Also, oftentimes horses are incredibly difficult to kill (just ask anyone who's shot a horse to euthanize it) , would the horse simply immediately tip over in most cases or would it be a slower process of collapsing over his own legs?
The question is not specific enough for the exact method of death for the horse. Did the death blow sever the spine? Or did it open up a main artery?
The "knight" of the time is not your plate covered tank of the movies. While still heavily armored compared to a peasant, your armor is probably under 50 lbs and perhaps as light as 20 lbs spread from your head to about your knees.
So purely weight wise, an athletic agile man could have the tumbling skills to dismount a dieing horse while armored for battle.
But the nature of horses and how they die/fall/collapse makes it likely that your hypothetical warrior could have a thousand different luck based outcomes.