Arsenal's Alex Oxlade-Chamberlain has revealed that he could be out of action until November.
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The England international, 20, was injured during Saturday's 1-3 Premier League defeat to Aston Villa.
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Manager Arsene Wenger said on Tuesday that he expected the midfielder to miss "at least" six weeks.
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But the player tweeted: "Gutted I'm injured so early in the season. But I'm going to be working hard to hopefully be back playing in three months."
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He is likely miss England's World Cup qualifiers against Moldova and Ukraine next month.
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Wenger said the player may not require surgery but added: "He will be out for at least six weeks. He has a posterior cruciate problem, a ligament which is stretched."
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Arsenal have already suffered injuries to key players such as Mikel Arteta this season while fringe players such as Andrey Arshavin, Denilson and Sebastien Squillaci have been allowed to leave.
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In June, Arsenal chief executive Ivan Gazidis said the club had the financial firepower to spend heavily in the transfer window, which closes on 2 September.
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But despite bids for Liverpool striker Luis Suarez and Newcastle midfielder Yohan Cabaye, French youth international striker Yaya Sanogo, 20, remains the only signing this summer.
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That has left some fans frustrated and many chanted for the Frenchman to strengthen his squad following the defeat to Villa.
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However speaking to reporters in Turkey ahead of Wednesday's Champions League play-off first-leg meeting with Fenerbahce, We defended his record in the transfer market.
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He said: "I would like to reiterate in the last 16 years we have been very successful with transfers.
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"If you look at the players who play tomorrow, they are top quality players, and you should never forget that."
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The Arsenal manager also defended his club's approach for Newcastle's Cabaye.
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Magpies boss Alan Pardew left the France midfielder, 27, out of his side's game at Manchester City on Monday following Arsenal's bid for the player.
Wow! That is a very sad one I must say. It wasn't something I was expecting @ all cos he is very important to the squad. Sad one!