Two late goals took Chelsea through to the semi-finals of the Carabao Cup, where they will face Tottenham.
Pontus Jansson turned Reece James’ low cross into his own net before Jorginho scored with a penalty five minutes later.
Chelsea, depleted by Covid-19 and injuries, gave senior debuts to teenagers Harvey Vale, Xavier Simons and Jude Soonsup-Bell.
But they were still able to secure a place in the last four – largely because of the introduction of seasoned players as substitutes.
Two of them combined for the opening goal, with N’Golo Kante superbly evading two challenges near the edge of the penalty area and finding James, whose delivery from the right forced the error from Jansson.
Jorginho and Mason Mount, who also came off the bench, were involved in the second goal.
After Brentford keeper Alvaro Fernandez brought down Christian Pulisic as the American latched onto Mount’s pass, Jorginho tucked away the resulting spot-kick.
Vale missed a great chance in the first half when, unmarked, he headed Cesar Azpilicueta’s cross straight at Fernandez.
Yoane Wissa earlier missed a similar chance for Brentford, heading Bryan Mbeumo’s cross straight at Blues keeper Kepa Arrizabalaga.
Brentford: Fernandez, Sorensen, Jansson, Pinnock, Canos, Baptiste (Onyeka 74), Jensen (Forss 82), Janelt (Norgaard 65), Henry (Ghoddos 74), Mbeumo, Wissa (Toney 65).
Subs not used: Cox, Thompson, Peart-Harris, Stevens.
Chelsea: Kepa, Azpilicueta, Chalobah, Sarr, Simons (James 65), Kovacic (Jorginho 45), Saul, Alonso, Barkley (Kante 76), Soonsup-Bell (Pulisic 45), Vale (Mount 65).
Subs not used: Bettinelli, Rudiger, Hall, Webster.