Kangaroos, unlike wallabies, have strong tails which they can lean on. Kangaroos can't sit down, but instead lean back on their tails. One sign of aggression is when they stand upright and lean far back on the tail, which means they are rearing to kick an opponent with their legs.
These guys look pretty small, but still if the guy had been kicked it would have been extremely painful. Not unlike getting an actual punch to the face.
He would've been lucky if it was just a 'got punched' level of extremely painful. Roos can disembowel you with their foot claws if they hit you a certain way.
"Gregor tried his hardest to convey his predicament. Tried to yell with all his might "I am a man! Help me!" But all that came out was a hideous, raspy bellow."