Mark O'Mahony has been sidelined at League One outfit Reading.
His boss Leam Richardson confirmed the setback ahead of a home game with Wycombe.
Richardson said: "Mark has got a knee problem.
"He will be out for a lengthy period of time.”
Richardson still expects to see O’Mahony play some part in what has been a frustrating season.
He added: “He's fully committed and Brighton are fully committed.
"As soon as he's back fit and available, he'll be in and around the squad and hopefully raring to get going."
The majority of O’Mahony’s appearances for Reading came under previous manager Noel Hunt, who gave him five league starts at the beginning of the season.
He fell down the pecking order when Richardson took on the role with Kelvin Ehibhatiomhan leading the line, and has only played 60 minutes across five league games under the current boss.
His minutes were further reduced when Jack Marriott was introduced back into the starting line-up against Luton in December.
Fellow Albion loanee Kamari Doyle is enjoying rather more success with Reading.
He has impressed in the No.10 role with three goals and four assists going into the weekend.