Millwall are set to be without striker Neil Harris for up to six weeks as he is suffering from a nerve problem.
The 31-year-old has helped the Lions recover from a slow start this season to climb up League One, winning their last three league matches.
But the former Nottingham Forest forward was ruled out of Saturday's victory at Leicester after aggravating the injury in training on Friday.
"It's immensely frustrating because I hate not being able to train or play," he said.
"The worst thing is there is no definite timescale for recovery."