Arsenal manager Arsene Wenger has confirmed that defender Gabriel Paulista will be out for up to two months with an ankle injury.
Midfielder Jack Wilshere, meanwhile, will miss the Premier League opener at home to Liverpool on Sunday with a knee problem.
Gabriel sprained his ankle in the Gunners' final preseason game against Manchester City on Sunday, but said on social media on Tuesday that the injury was "not a big problem."
However, Wenger told Arsenal's official website: "Gabriel has an ankle problem, and will be out for six to eight weeks."
He said that prognosis was "a bit better than we feared at the start."
The news adds urgency to Arsenal's attempts to sign a new centre-back in the transfer window, with Per Mertesacker out with a long-term knee injury and Laurent Koscielny set to miss the Liverpool game because of a lack of match fitness.
Wenger said Mertesacker would be out for four to five months, but added that injured forward Danny Welbeck could return within that timespan after having initially been ruled out until February.
"We had very good news on Danny's latest scan and Per is at the moment off his feet for three weeks, so of course if is a bit boring for him," he added.
"He is with his family in Hannover, but on the football front he is very frustrated. We think it will be four to five months for both players."
Wilshere picked up a minor patella problem during Arsenal's preseason tour of the United States, and although he is back in training this week he will not be in the squad to face Liverpool.
However, the England midfielder told Sky Sports he expected to be back on the pitch soon.
"I picked up a little knee injury, nothing serious, and now I am back in light training and am looking forward to the start of the Premier League," he said.
Wilshere, who has struggled with a number of ankle problems and other injuries, added: "I've been thinking for a few years: 'this has got to be the year where I don't get injured.' Hopefully this year I get a long run."