You could argue the keeper shouldn't come out, which is obvious.
You can also argue that the defender if he kept running he would have stopped it, he jogs back until it hits the post then accelerates.
You can also say because of Zlatan being unorthodox in his approach the defender assumed it was going in so already stopped running, and that bought the chance a split second more to go in. (if he shot lower and it was clearly trickling in the defender would have sprinted to stop it).
Poor game though.