The Royals boss, who watched his side throw away a four-goal lead, insisted that conceding on the stroke of half-time is what cost them the game.
"It came down to the goal we conceded before half-time. We went in at 4-1 and we weren't happy or comfortable," he said.
"It was suicide what went on in the second half and extra time. You play each game on its merits but it was extraordinary.
"I've never seen anything like it. We have to dig deep and move onto the next game versus QPR."
Jason Roberts, Laurent Koscielny, Mikele Leigertwood, Noel Hunt, and Pavel Pogrebnyak all found the net for Reading, but goals from Olivier Giroud and Laurent Koscieln, combined with a Marouane Chamakh brace and a Theo Walcott hat-trick fired the Gunners into the next round.