The duo came off injured in the first half of the 3-2 Premier League victory away at Nottingham Forest last weekend.
Brighton now have an extensive injury list, which features defenders Adam Webster and Pervis Estupinan along with the versatile pair of Solly March and James Milner, plus forwards Danny Welbeck, Julio Enciso and Kaoru Mitoma.
Speaking ahead of the Europa League clash away at AEK Athens on Thursday, head coach De Zerbi said: “Igor [Julio] can play. Ansu [Fati] and [Tariq] Lamptey are both injured. We have lost them for a long time, I think.
"But the focus is on the players who can play tomorrow. We will have to compete. I don’t know if we will win, but we will fight to win the game.”
Both Fati and Lamptey were making just their respective second starts of the season on Saturday.
Brighton will have to pick their strongest team away in Greece with Group B still in the balance with two games left to play.
The Seagulls will be able to field captain Lewis Dunk and midfielder Mahmoud Dahoud in Europe, but they are both suspended for the visit to Stamford Bridge on Sunday.
Chelsea have also suffered injury problems this season and against Brighton will be without both skipper Reece James and Marc Cucurella, who are suspended.
Mauricio Pochettino is expected to be without eight players of his own for the upcoming Premier League match, with Brighton currently two places and six points above Chelsea in the table.