Albion manager Micky Adams fears Lee Steele could be out of action until the middle of March.
Steele was due to see a specialist yesterday about the right knee he damaged in a car crash last week.
Adams said: "There is obviously a concern. The knee is very lax. There is no stability in it.
"It depends what the specialist finds. At best it will be three to four weeks, at worst three months."
The timing of Steele's absence could not have been worse. The former Shrewsbury striker had been lined up to replace the banned Bobby Zamora at Barnet on Boxing Day.
Steele is certain to miss at least six matches squeezed into 19 days over the busy Christmas and New Year period.
Meanwhile, Adams is demanding Danny Cullip proves his fitness before Friday night's Withdean sell-out against Exeter.
The influential central defender missed training last week with a recurring calf problem but still made the starting line-up for last Saturday's defeat at Mansfield.
"I have got to be careful," Adams said. "I don't want any chances taken with it.
"Missing a week's training and just playing games is no good to the team or the individual.
"I am not saying he let the side down at Mansfield, but I don't think he was 100 per cent. I expect him to do some sort of training this week."
Rightback Paul Watson suffered a whack on the neck at Mansfield, but he should be fit for the Exeter match.
Only a few tickets are left for Albion's noon showdown with Southend on New Year's Day. Tickets are now on sale for the home game against Lincoln on January 6.
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