Mo Salah started for Liverpool in the run-up to their trip to the Amex.
But they will head into their Premier League date with Albion on the back of a defeat.
Goals after the break from Josip Ilicic and Robin Gosens secured Atalanta a very impressive 2-0 win at Anfield in the Champions League.
That puts more pressure on fixtures next Tuesday, when Liverpool host Ajax.
Jurgen Klopp made five alterations to the XI which had kicked off the impressive win over Leicester City.
Klopp is unhappy his side must tackle the Seagulls in the 12.30pm kick-off on Saturday.
As well as Salah, he brought in Divock Origi, Rhys and Neco Williams and Konstas Tsimikas.
James Milner moved from right-back into midfield alongside Gini Wijnaldum and Curtis Jones.
Fabinho began on the bench, as did Diogo Jota, Roberto Firmino and Andy Robertson.
Neither Thiago Alcantara nor Jordan Henderson were involved.
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Liverpool made four substitutions immediately after going behind on the hour.
Winjaldum and Salah were given a rest for the last 30 minutes with Origi and Tsimikas also going off.
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Jota, Firmino, Fabinho and Robertson all went on - but Atalanta doubled their lead soon after.
Takumi Minamino went on for Matip late on with Fabinho dropping into defence.
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