The injury (which occurred during the NRL fixture between McKinnon's club, Newcastle Knights, and the Melbourne Storm) came from a three-man tackle on McKinnon, which resulted in the young forward landing on his head, breaking his C4 and C5 vertebrae in the impact.
Recent reports have confirmed that McKinnon has undergone emergency surgery to remove a disc and fuse the dislocated vertebrae, and he has been placed in an induced coma before further tests will reveal the full extent of damage to his neck and spinal cord (although it was encouraging to read that he does have movement in his right arm).
Rugby league is, in my opinion, the most consistently exciting spectator sport there is, a mixture of sublime skills, pace, strength, and incredible physicality. Those at the top level of the game are quite simply outstanding athletes, but it takes a fair bit of courage to play the game at any level, and the injury sustained by Alex McKinnon certainly serves as a cautionary reminder of the occasionally terrible, but thankfully rare, consequences of such an intense contact sport.
Those who know the game far better than I, may question the effect of the three-man tackle rule change, where the third man can no longer tackle the player's legs, and whether this affected how McKinnon was driven to the ground. Also, it seems as if McKinnon ducks his head slightly just before hitting the playing surface: did this arguably worsen the impact of the tackle? For me, having watched the incident a couple of times at full speed and in slow motion, it is probably just an awful accident; one of those things that is unintentional and unavoidable simply because it happens so fast.
In fairness though, all that really matters here is for this brave young man to make as full and swift a recovery as possible, and I hope that is exactly what happens. All the best Alex.