Fabian Delph came off with a setback against Watford. He was substituted in the 81st minute for Allan.
Lampard has admitted that the experienced midfielder suffered a muscle injury - and is sidelined for the Brentford clash.
“Fabian won’t be fit for Brentford,” said the Blues boss.
“We’ll see for the rest of the season. It’s a small injury, a muscle injury. Enough to keep him out for this game.”
There is better news, however, as Ben Godfrey is once again in full training and could be in the match-day squad.
As could Donny van de Beek, who has missed the past four games with a groin injury.
Lampard said: “Ben Godfrey is very close. Whether he’s fit for this game or not, but he’s possibly going to be in the squad.
“Donny van de Beek is getting closer. We’d like to think he might be involved on Sunday, if not Thursday (against Crystal Palace).
“There’s not much change after that.”
Yerry Mina will once again be sidelined with a calf injury.