Lukaku has been brought back in by Thomas Tuchel
"I don’t think it was very clever for him to speak about Tuchel and his tactics." Gallas told Genting Casino, adding that "Players have to be very careful about what they say. There is so much media interest in absolutely everything."
"He has received his punishment and learned from it. The most important thing is how he responds," the Frenchman continued.
"He has to be focussed and ready for his team. He needs to start scoring goals for Chelsea."
Lukaku has scored once in three games since returning to Tuchel's side, finding the net in the FA Cup victory over Chesterfield, and Gallas – who won two Premier League titles with Chelsea – suggested he needs to follow it up with more regular goals.
"The people at Chelsea will only forgive if he starts performing at the level we have come to expect from him," he said.
"Chelsea fans will not forget what he said unless he starts scoring goals and delivering match-winning performances.
"When you’re [a top player] you have to adapt to any system and you have to score goals. That’s what is expected from you."
Gallas has said Lukaku needs to get his goalscoring touch back
The goal against Chesterfield was Lukaku's eighth in all competitions since returning to Chelsea, seven years after his first spell with the club came to an end.
However, he has some way to go before matching the tallies he posted in two years as an Inter Milan player.
The 28-year-old scored 64 goals across two seasons in Italy, prompting Chelsea to part with £97.5m to bring him back to Stamford Bridge, with 47 of those goals coming in Serie A.
He started both legs of the Carabao Cup semi-final against Tottenham Hotspur, completing 90 minutes on both occasions, but failed to find the net in either game.