Brighton striker Stefanos Tzimas faces an extended spell on the sidelines.
Tzimas, 20, sustained an ACL injury last season in the 4-3 home loss to Aston Villa last December
The Greek attacker missed the remainder of the season and continues his rehabilitation this term.
The 22m signing from FC Nürnberg had been making progress and there was hope of a return in the coming weeks after the next international break.
Stefanos Tzimas setback
However, head coach Fabian Hurzeler, speaking ahead of Brighton's third round Carabao Cup clash at Manchester United on Wednesday, confirmed that Tzimas suffered a setback.
"Nothing big but it takes a bit longer than we expected," said the German.
"He is in a good mood and tries to work hard on the pitch. We try to help him off the pitch and hopefully we will see him soon."
It's a blow for the Seagulls who sold last season's leading scorer Danny Welbeck to Chelsea last month.
Evan Ferguson is sidelined
Striker Georginio is also injured - although he is said to be "close" - and Ireland international Evan Ferguson remains sidelined.
Ferguson sustained ankle damage while on loan at Roma last term and continues his recovery.
Despite the problems, Brighton have been in goalscoring form on the pitch. Hurzeler's men have 13 goals from four Premier League matches, the highest in the division.
The Seagulls also fired four past Norwegian club Tromso in their Conference League play-off second leg.
Hurzeler, speaking after the 5-0 win at Coventry, said, “I mentioned at the beginning of the season that it's not about having one good goalscorer.
“It's about having this flexibility of having many goalscorers, of having a lot of players who have the desire to score.”
Kostoulas or Promise for United?
Greek striker Charalampos Kostoulas - who opened the scoring at Coventry last Sunday - or Canada international Promise David could start up front for Albion against Manchester United.
Promise is yet to start for Brighton this term following his loan move from Belgian club Union SG.
Georginio continues to be assessed having missed the last three Premier League fixtures.