The Dutchman, who joined in a £40 million deal from Ajax, made his debut in the 2-1 victory over Nottingham Forest on Saturday.
However, the versatile defender suffered a suspected knee injury five minutes into the second half and was replaced by Takehiro Tomiyasu.
Further examinations have shown that Timber may have picked up a significant knee ligament injury that could see him miss the next few months of action.
There were also initial concerns that the Netherlands international had damaged his anterior cruciate ligament, but that has not yet been confirmed.
Timber’s injury will come as a huge blow to Mikel Arteta, who is already dealing with the absence of first-choice striker Gabriel Jesus.
Arteta saw injuries to key players derail his side’s Premier League title challenge last season, and it seems they have picked up where they left off this term.
Timber is a two-footed defender capable of featuring in several roles across the backline, and the Gunners will miss his versatility in his absence.
The 22-year-old will likely miss games against Manchester United, Tottenham Hotspur and Manchester City while in the treatment room.
Those fixtures will all take place within the next two months, while Arteta’s side also have Champions League commitments during that period.
The good news for the Arsenal supporters is that Oleksandr Zinchenko has been declared fit to play – although he was not in the squad that defeated Forest last weekend.
However, the Ukrainian’s return could be fast-tracked following Timber’s injury, with the Gunners gearing up for a London derby against Crystal Palace next weekend.
Kieran Tierney could also be an option, but his long-term future at the Emirates is far from certain, with playing time hard to come by.
The Scottish full-back is the subject of interest from Newcastle United and could depart Arsenal before the transfer window closes.
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