Bayern Munich were already winning 3-0 when Arjen Robben entered the fray against Arsenal. Usually, bringing on the third or fourth best attacking midfielder on the planet when you're up 3-0 would be grounds to constitute criminal behavior, but it's a good thing this is the Champions League and goal differential matters.
So Arjen Robben proceeded to score because this is the natural order of things.
It was 37 seconds from his subsitution to his goal, as Robben scored with his first touch of the match. Furthermore not only did Arjen Robben score a terrific goal, but it was once again David Alaba coming out of attacking midfield and steaming through the Arsenal defense that was the spark for this goal. Similar to his amazing striker just before halftime, David Alaba has spent this match popping up in critical positions and driving Arsenal mad.
And what's one more goal to add to the three already scored?