The Brazilian had not scored since finding the net in a Europa League tie against Steaua Bucharest in October 2010, but he provided the decisive touch of this third-round replay with an 18th-minute header.
“It’s good to score, I don’t score many and I helped the team, that’s the main thing,” Lucas said. “The main thing was to get through to the next round and we did it.”
Liverpool manager Jurgen Klopp was more pleased with the result than the performance. “The longer you don’t score the more exciting it can get and you saw that when they started getting really direct, we are not used to it and they hadn’t played too often together — the back-line,” he said.
“Okay, they had a few chances but so did we, especially in the last 20 minutes where we controlled the game.
“Maybe it wasn’t the most exciting game but for us it it’s very important as...it’s another game for a few of the young boys, so that’s all good.”
League Two Plymouth came agonisingly close to an equaliser when Jake Jervis volleyed against the post 15 minutes from time and, although Divock Origi saw his late penalty saved by Luke McCormick, Liverpool move on to a home tie with Wolves in the next round.
The Premier League side chose to omit Joel Matip as the row over the defender’s eligibility rumbles on.
Klopp felt he could not risk playing Matip with Fifa unable to say whether he is free to play after Cameroon tried to call him up for the African Nations Cup.
But Origi, Daniel Sturridge and Philippe Coutinho all started in a formidable looking frontline as Klopp named a stronger line-up than the one held by their Devon opponents on Merseyside 10 days earlier.
Plymouth only had 23% possession at Anfield and the tie quickly developed a similar pattern as Liverpool set up camp in the Pilgrims’ half.
Sturridge’s shot lacked accuracy and Coutinho scuffed an inviting effort when well placed at the far post.
Liverpool also saw penalty appeals waved away after Sturridge fell under Oscar Threlkeld’s clumsy challenge, but the visitors were ahead after 18 minutes.
Lucas had scored only six goals in nearly a decade at Liverpool — and none for over six years — but it was the Brazilian who stole in unmarked at the near post to head home Coutinho’s corner.
Moments later Lucas, the Liverpool captain for the night, stopped Ryan Donaldson from claiming an instant equaliser after Loris Karius had pushed out Graham Carey’s well-struck drive.
It was a sign Plymouth were determined not to slip quietly from the FA Cup and the hosts played their best football of the entire tie as the Liverpool defence had to scramble on more than one occasion.
Early in the second half, Irish midfielder Carey was at the heart of an incisive one-touch move which ended with David Fox firing from distance and Karius gathering on his line. But Liverpool slowly re-established control with Sturridge finding the side netting and Origi forcing a save from McCormick. Klopp was relaxed enough to withdraw Coutinho and send on Harry Wilson for his Liverpool debut.
But Liverpool had an almighty scare when Jervis was left unattended from Sonny Bradley’s knock down to volley against the post with Karius beaten.
Origi failed to double Liverpool’s lead after Yann Songo’o had fouled Alberto Moreno in the penalty area, sending his shot straight at McCormick.
But Plymouth can take pride from a committed performance safe in the knowledge that their FA Cup exploits this season have earned the club an estimated £1m (€1.1m).
McCormick, Threlkeld, Songo’o, Bradley, Purrington, Fox, Jervis, Donaldson (Tanner 67), Carey, Slew (Bulvitis 80), Garita (Rooney 72).
Karius, Alexander-Arnold, Gomez, Lucas, Moreno, Coutinho (Wilson 65), Stewart, Ejaria, Sturridge (Ojo 76), Origi, Woodburn.