Mikel Arteta expected Bernd Leno's injury to be more significant but revealed Arsenal had suffered another injury blow to striker Gabriel Martinelli.
Goalkeeper Leno appeared to have suffered a serious injury when he was taken off on a stretcher during Arsenal's 2-1 defeat to Brighton and Hove Albion on Saturday following a collision with Neal Maupay.
However, the Gunners announced that Leno had suffered only a moderate ligament sprain in his right knee and he is anticipated to return to training in four to six weeks.
Arteta told a media conference: "We thought it was going to be a more significant injury.
"We don't know the extent of the injury still but we know at least the cruciate ligaments are not damaged. That was the biggest fear at the start."
Granit Xhaka (ankle) and Sokratis Papastathopoulos (thigh) are both back in full training but Arteta is unsure whether either will be available for Thursday's trip to Southampton.
"They haven't trained fully with the team, let's see how they are tomorrow. They are not yet ready," he added.
One player whom the Gunners will not be able to count on is Martinelli, who is set for months on the sidelines after injuring his knee in training.
"We had some really bad news yesterday [Monday] - in training, he got hit by another player," said Arteta of Martinelli.
"He damaged his knee. The doctors are assessing the extent of the injury, but it's not looking good at all. It's looking like months."
In addition to their injury issues, Arsenal have the complication of needing to extend the stays of loanees Pablo Mari, Cedric Soares and Dani Ceballos - as well as that of David Luiz, whose contract is due to expire - by midnight (local time) on Tuesday.
"It's very complicated the timing of it and some of the legal issues that are involved," explained Arteta.
"The club is trying to finalise everything. We have to do it, and even more with the injuries that we have. We are trying to resolve this. I'm confident we can do it."