Osei Sankofa, who has been filling in at either right back or centre-half while M'Voto was out with his hip injury, is now a major doubt himself for Saturday's match at Roots Hall against Bristol Rovers.
Scans taken Monday confirmed the knee injury was serious and he'll now visit a Harley Street specialist on Wednesday for a full diagnosis.
Blues physio John Stannard told www.southendunited.co.uk: "Osei's knee really swelled up after Friday's match with Southampton. His knee has caused him some discomfort for a few weeks but we only learned of the problem at the weekend. A scan on Monday showed that he has a serious knee injury and we will await the specialists' view tomorrow as to how we move forward with treatment."
M'Voto's return from Wearside is very timely.