Tamika Mkandawire is out of action for at least two weeks with injury.
Orient's 25-year-old defensive stalwart limped off during the 1-0 home defeat to Walsall, last Saturday.
Tests have revealed a strained medial ligament.
Orient chairman Barry Hearn has said that boss Geraint Williams is looking for a replacement.
Mkandawire's injury comes as top scorer Adam Boyd gears up for a return from a month long lay-off.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article