The defender, who arrived through a loan with an option to buy deal in January, was injured against Manchester City on Wednesday.
Now Sky Sports reports that Mari is done for the season with an ankle injury.
Granit Xhaka is another who is set to miss the Saturday game against Brighton and Hove Albion.
The club is likely to make an official statement on both injuries in the next day or two.
Mari's injury may impact his hopes of earning a permanent contract with the Gunners as well.