The England midfielder starts again up top alongside Raheem Sterling in a presumed 3-5-2 system from Frank Lampard, with the likes of Joao Felix and Mykhailo Mudryk confined to the bench.
Reece James and Mason Mount will not play again this season while top scorer Kai Havertz has also been ruled out of the clash.
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They join Kalidou Koulibaly and Armando Broja in the treatment room. James injured his hamstring against Real Madrid, playing through the pain in a huge Champions League match but ultimately ending his season early.
Mount is having minor surgery on a painful pelvic bone injury that needs four weeks to recover, while Havertz has a minor knee issue.
Koulibaly came off with a hamstring problem in the first leg against Madrid a fortnight ago, while Broja is out with a long-term knee injury but the rest of the squad is in good shape.
Lampard previously admitted that his players were lacking in fitness and “capacity” but has had a full week to prepare for Brentford.
It is the first time that Lampard and his coaches have had a clear run having had games every three days since coming in to replace Graham Potter.
They have lost all four matches but did see their side compete well against Madrid last week. Chelsea played in a 3-5-2 formation then with Gallagher up front and do so again here.
Chelsea starting XI: Kepa, W Fofana, Thiago Silva, Chalobah, Azpilicueta, Fernandez, Kovacic, Chilwell, Kante, Gallagher, Sterling
Subs: Badiashile, Aubameyang, Pulisic, Joao Felix, Loftus-Cheek, Mudryk, Mendy, Ziyech, Madueke
