Mind if I share not my experience but a friend's?
My friend, we'll call her May [not her actual name] was on holiday, someone was looking after her horses while she was away. All of a sudden May gets a phone call... there's been a big storm and there are three injured horses on her property [they have a few, May's mother breeds Arabs]... and May's project pony, a beautiful Welsh D called Peggy [actual name] had really bad cuts on her hind legs, one right the way to the bone. Was found upside-down in a drainage ditch with mud coating the legs.
Vet was out and treated. By the time Peggy was found she had been in the ditch injured for some time, the wound was not brand new and fresh. Vet's prognosis was, Pegs would be lucky to ever be sound enough for light riding. Basically she was facing a future as a broodmare, IF she even survived. Infection was a very real danger.
Peggy's recovery was long and often frightening, and she re-injured herself cutting open the healing leg a few times, but one year to the day after the injury happened, May saddled up and RODE. Now, another year or two later, Peggy is doing everything a normal horse would do with absolutely no lasting ill effects. There's barely even a scar and trust me it was a HUGE wound.
Whatever you do, don't lose hope. Pegs severed her extensor too and she has returned to full dressage work and low-level jumping, is actually more advanced now than she was before she did the injury.