Without magic being involved and with a normal human (albeit a 7-9ft one) could they heal if the back of their lower leg were cut open across the muscle grains or would this be a permanently crippling wound?
Yes, the wound would heal over but they might never walk properly again. It depends how deep the cut is. Either way, healing would take time, and during that time the muscles would waste away. You might be able to build them up again, but that would take even more time. You're talking months of recovery time if the character is to regain full muscle strength after a deep cut like that.Without magic being involved and with a normal human (albeit a 7-9ft one) could they heal if the back of their lower leg were cut open across the muscle grains or would this be a permanently crippling wound?
If you catch it fast enough you can, with correct footplacement and fixing it in place heal most of it in such a way that you can walk again. You basically fixate the thing in place and wait until scar tissue grows enough to hold it in place. A better solution is to cut open your leg, sow the two parts back together and close the thing up again.
The surgical method would require a level of anesthesia and operating room technology not seen before the twentieth century to do it safely, but at least in theory, the correct foot placement and fixing it in place could be done at the pre-modern level. What it would take is a practitioner with great skill at manipulating the human body and knowledge of how to do it. Such things have existed for millennia. Practitioners of the past could develop an uncanny (to us) sense of how to do it all by feel. That's an art that's lost when we have x-rays and other imaging technology to rely on.The thing is though that this very much all depends on 20th century knowledge of medicin. If I'd done the same thing 50 or 100 years earlier I'd probably be limping for the rest of my life.