Arsenal boss Arsene Wenger will assess the extent of Gabriel's injury before deciding whether to sign a new defender.
The Brazilian was stretchered off after a last-minute challenge with Kelechi Iheanacho in the Gunners' 3-2 friendly win over Manchester City.
And Wenger has now conceded that, with Per Mertesacker also out and Laurent Koscielny not fully fit, he may have to turn to the transfer market.
"I've said many times that we are in the market," he said, in quotes reported by Arsenal's official website.
"Of course it's not an easy market for us. Let's hope we have good news from Gabriel.
"Laurent Koscielny is back in training [on Monday] but he might be short for next week [Premier League opener against Liverpool].
"If Gabriel is out, Per Mertesacker is out and Koscielny not ready - that's a lot of players."
Arsenal have already brought in 20-year-old Rob Holding from Bolton Wanderers, but are short of senior options