<text>  Arsenal legend Martin Keown believes Per Mertesackers omission from Arsenals starting line-up for Sundays win over Everton could be a defining moment in changing the clubs back four. January signing Gabriel Paulista was preferred to the German in defence, with Gunners boss Arsene Wenger telling BT Sport that the change was made in order to give Mertesacker a rest. Sportsmails Keown, speaking as a pundit on BT Sport, reacted to the news of Mertesackers absence for the crucial game at the Emirates saying: I dont you can point the finger at one player after the Monaco game. Its a big chance for Gabriel now, though. Martin Keown believes Per Mertesackers omission could be a defining moment in Arsenals defence . Keown third right was joined by Steve McManaman far right and Michael Owen second left on BT Sport . I think it could be a defining moment in changing Arsenals back four now, for sure. Mertesacker, with the experience he has in the game, should have been calling these young players back the other night, the two full-backs were just drifting up the pitch and not coming back. When hes been left isolated one-on-one he struggles due to his lack of pace too. Gabriel is keen to defend so hell be relishing this opportunity to stamp a claim for a first-team spot. The German defender centre looks on despondently after Arsenal concede a third goal on Wednesday . Arsene Wenger has come under a barrage of criticism after Arsenals shock defeat to Monaco on Wednesday . Wenger, who has come under fierce criticism after Wednesday nights 3-1 defeat by Monaco in the Champions League, maintained he is still fully committed to Arsenal, urging supporters to stand by the team ahead of the 2-0 win over Roberto Martinezs men. In an interview with BT Sport he said: We have always been united at Arsenal so I ask the fans to stay behind the team and show how strong you believe in the team even after Wednesdays disappointing result. In the last 25 years Ive always finished in the top three four and that consistency has given me belief and confidence to continuing to manage at the highest level. Im as committed as Ive ever been to deliver success to this team. 