However, it was one-way traffic after the restart as Robert Lewandowski, Thomas Muller and a double from Thiago Alcantara put Arsenal on the brink of a seventh successive second-round exit from the competition.
Former Gunners defender Keown told BT Sport after the final whistle: "I feel for Arsene Wenger. It's almost embarrassing. Outclassed, outplayed. They did it at a canter.
"Wenger has to be seriously considering his future now because it's embarrassing. I can't ever say I'd like to see him go - this is his lowest point ever as Arsenal manager."
The result means that Arsenal have become the first English team to concede 200 goals in the Champions League.