article_snippet: Sportsmail's Martin Keown Sportsmail's Martin Keown believes Per Mertesacker's omission from Arsenal's starting line-up for Sunday's win over Everton 'could be a defining moment in changing the club's back four'.
article_snippet: Sportsmail's Martin Keown believes Per Mertesacker's omission could be a defining moment in Arsenal's defence .
article_snippet: I think it could be a defining moment in changing Arsenal's back four now, for sure. '
highlights_snippet: Per Mertesacker was a headline omission from Arsenal's starting line-up .
  
highlights_snippet: Sportsmail's Martin Keown believes it could be a defining moment in changing Arsenal's back four .