Keown saw his former club shock Barcelona 2-1 on Wednesday.
He wrote in his column for the Daily Mail: "Lionel Messi is so hard to mark because he drops so deep into the midfield and no-one can pick him up. You are taught as a defender to never run that deep with a player - it is against the rules - but you have to rip up the rule book with Messi.
"Arsenal did just that in the second half.
"Johan Djourou and the excellent Laurent Koscielny made a conscious decision to go deep with him after the break and it worked as the Argentina playmaker was no longer a threatening, unmarked spare man.
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