The key defender's hamstring injury is worse than originally feared and he is not expected to play any part in the Gunners' Champions League and Premier League fixtures this week

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By Wayne Veysey | Chief Correspondent Bacary Sagna will miss Arsenal's crunch clashes against Napoli and Manchester City understands.The defender's hamstring injury, which has kept him out of the Gunners' last two matches, is believed to be worse than originally feared.Manager Arsene Wenger initially claimed Sagna had "a slight hamstring problem" but Goal understands the Frenchman is not expected to play any part in the two showdowns spread over just four days.Carl Jenkinson will keep his place in the team for Wednesday night's decisive Champions League trip to Napoli, where Arsenal only need to avoid a three-goal defeat to reach the knockout stages. Victory would guarantee top spot in the highly competitive Group F.Arsenal then travel to title rivals City on Saturday lunchtime to end a congested programme that has infuriated Wenger.The likely absence of Sagna is a blow for the Premier League leaders, even though Jenkinson has proved himself an increasingly able deputy at right-back over the last two seasons.Sagna has been a key figure in an Arsenal defence that has looked more solid this season than at any point in the eight years since the club last won a trophy.In the final year of a contract that is due to expire next summer, the 30-year-old has made 19 appearances, starting all seven of the club's Champions League matches and 11 out of 15 Premier League games.