Millwall Football Club can confirm that Byron Webster has suffered a serious knee injury.
The defender has ruptured a cruciate ligament and underwent surgery on Thursday.
Webster, 30, will now begin a lengthy period of rehabilitation at Calmont Road as he looks to return to fitness during 2018.
"This is a major disappointment, first and foremost for Byron, whose form at the start of this season has been outstanding," Neil Harris told millwallfc.co.uk.
"As well, it is of course a disappointment for the group. However, we will grow stronger because of it. Others will now have to step up in Byron's place and take their opportunity.
"We don't want to put a definitive timescale on his recovery but it will be long-term. Seeing Byron back before the end of the season would be a huge bonus."