No timescale has been placed on his return -- that will depend in part on the success of his operation -- but there are fears he could be sidelined for the majority of the 2023-24 campaign.
Timber posted on social media: "Gutted to share my injury is more serious than expected, especially after the warm welcome I've received. I wanted to repay you on the pitch, which will not be possible for the forthcoming period. Thanks for making me feel at home and see you at The Carpet."
Arsenal said in a statement: "Further to detailed assessments with consultant specialists since Saturday, we can confirm that Jurrien has sustained an injury to his anterior cruciate ligament in his right knee.
"Jurrien will undergo surgery in the coming days and will be out of action for a period of time.
"The support and expertise from our medical team and everyone at the club will now be focussed on Jurrien's rehabilitation programme, to ensure he recovers well and is back playing as soon as possible."
The news will come as a significant blow to Arsenal boss Mikel Arteta, who started Timber at left-back in the Community Shield victory over Manchester City and against Forest.
Arsenal might reenter the market to strengthen their defensive options, given the severity of the injury.
Another left-back option, Kieran Tierney, has interest from Real Sociedad and Newcastle, with Arsenal willing to listen to offers, while Oleksandr Zinchenko is nearing a return to action after a calf problem. Nuno Tavares is in talks to join Nottingham Forest.