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Barcelona loanee Fati was missing from the squad alongside January signing Valentin Barco at Brentford on Wednesday (April 3).
However, the pair are both featured on the team sheet to face Arsenal today (Saturday, April 6 - 17:30 BST).
"Fati and Barco, I didn't decide to play with them in the squad for this game," Roberto De Zerbi said, after the 0-0 draw at the Gtech Stadium.
"Fati is working very well but from him I have a different expectation.
"I don't change my opinion about Ansu Fati - he is one of the best talents in the world for his age.
"Our responsibility is to help him improve - to improve his performances, his physical condition, his mentality.
"To play in the Premier League, you have to be stronger mentally and physically. When we shows us the right condition, he will be ready to play for us and I will be happy."
Five changes have been made to the starting XL.
Tariq Lamptey, Pervis Estupinan, Jakub Moder, Julio Enciso and Danny Welbeck all start. They replace Igor Julio, Adam Lallana, Facundo Buonanotte an Joao Pedro.
Pedro's minutes are being managed as he returns from a thigh injury.
Striker Evan Ferguson (ankle), midfielder Billy Gilmour (knee) and defender Adam Webster (muscular) had all been ruled out for the clash against the Premier League title chasers.
The Seagulls were already without Kaoru Mitoma (back), Solly March (knee), James Milner (thigh) and Jack Hinshelwood (foot).
Brighton: Verbruggen, Lamptey, van Hecke, Dunk, Estupinan, Baleba, Gross, Adingra, Moder, Enciso, Welbeck.
Subs: Igor, Pedro, Lallana, Barco, Steele, Ansu Fati, Veltman, Buonanotte, Peupion
Arsenal: Raya, White, Saliba, Gabriel, Zinchenko, Odegaard, Jorginho, Rice, Saka, Havertz, Gabriel Jesus
Subs: Ramsdale, Partey, Smith-Rowe, Martinelli, Nketiah, Kiwior, Tomiyasu, Trossard, Fabio Vieira