Gunners manager Arsene Wenger told a press conference on Tuesday that the 20-year-old will be out for at least six weeks with a knee injury.
The player himself later tweeted that he hoped to be "back playing in three months".
Oxlade-Chamberlain limped out of Arsenal's 3-1 home defeat by Aston Villa on the opening day of the Premier League campaign on Saturday.
Reports suggested he could be set for an even longer spell of rehabilitation, but Wenger moved to calm fears of a serious injury as he spoke ahead of the Champions League play-off first leg against Fenerbahce on Wednesday.
"He will be out for at least six weeks. He has a posterior cruciate problem, a ligament which is stretched," Wenger said.
"We don't think it will need surgery. He is consulting tonight, but it will be at least six weeks out."
But the player, who did not travel to Turkey and was due to see a specialist on Tuesday evening, later gave a much more pessimistic update on his official Twitter feed.
"Gutted I'm injured so early in the season," he wrote, "but I'm going to be working hard to hopefully be back playing in 3 months. Thanks 4 all ur support."
Oxlade-Chamberlain will miss September's crunch Group H World Cup qualifiers against Moldova and Ukraine.
The midfielder also stands to miss the October dates against Montenegro and Poland as Roy Hodgson's team bid to reach next summer's finals in Brazil, as well as the important early league and European games in Arsenal's campaign.