Forrester was denied securing the biggest giant-killing act in what is becoming a season of sensational cup upsets after Fernando Torres' cute finish snatched a late 2-2 draw in the fourth-round tie at Griffin Park.
European champions Chelsea were simply bullied by their opponents throughout the lunchtime contest, with Forrester's penalty looking like the winner before Torres struck seven minutes from time.
Marcello Trotta had fired the hosts in front moments before half-time but Oscar equalised for Rafael Benitez's Blues with a delightful strike.
Brentford's hopes of victory in the replay at Stamford Bridge in three weeks' time have already been written off, but man of the match Forrester insists they could still spring a surprise.
"Yes, but we will still be the underdogs which we will take," he said. "We will go there and enjoy. We will have a different game plan again.
"For the replay, we can say we have done it once, we got a draw, we maybe should have got more. We won't be scared, we weren't scared here. We came out to let our football do the talking.
"To come out 2-2 against the champions of Europe and be half disappointed shows how far we have come. We have gone ahead, got seven or eight minutes left and we let a goal in.
"Man for man, they are better but, collectively, we can show we have got the talent and determination.
"We all know what we have got in the changing room. We have got some good players even in the third tier of English football. We didn't let the occasion overawe us."