The defender limped out of last weekend's 3-1 defeat to Southampton with the problem and is set for six weeks on the sidelines as a result.
Manager Steve Tilson has further defensive worries with loanee Jean-Yves M'Voto still struggling with the hip injury that has ruled him out of the last five games.
The Sunderland man is expected to play some part, though, while Alan McCormack should definitely be involved after shaking off an ankle problem.
George Friend will play the last game of his loan spell from Wolves, although Tilson is keen to extend his time with the Shrimpers and is speaking with Mick McCarthy about doing so.
Tilson will be keen to arrest a run of three home games without a win against the Pirates, with United's last home success coming against Leyton Orient on September 4.