I judge the tackle to fair and played on. But why I post the question was the decision in our Law book. Decision 4
A tackle from behind, which endangers the safety of an opponent, must be sanctioned as serious foul play.
When does one judge it to be risking the safety of the opponent. Some say any tackle from behind is dangerous. I like to judge as you pointed pot OKO careless/reckless/dangerous. These 3 basic judging tools do help.