The Gunners were put to the sword by Monaco leaving their Champions League future in the balance.
Keown said: "This will be one of Arsene Wenger's most disappointing nights in the Champions League.
"It's against his former club and there was such expectancy on Arsenal tonight.
"They were expected to win this comfortably. Such a turnaround.
"I'm absolutely stunned.
"Oxlade-Chamberlain looked like he was carrying the fight for Arsenal but that sucker punch at the death sent Wenger storming off down the tunnel.
"Absolutely stunning. Arsenal guilty of such exuberance, giving the ball away and being punished yet again on the break.
"Arsenal guilty of such naivety.
"Now, Arsenal will be fighting to remain in this competition."