Steven Naismith scored a hat-trick as the Toffees easily saw off Jose Mourinho's ailing Blues at Goodison Park in the Premier League earlier in the season.
However, since Guus Hiddink took over from Mourinho in December, the London club are unbeaten in their last 12 Premier League matches, and the London club's only two defeats in their 17 matches under the Dutch manager in all competitions have come against French giants Paris Saint-Germain in the Champions League.
Lukaku knows that Saturday's FA Cup quarter-final clash against Chelsea is likely to bear no resemblance to the comfortable league victory in September, and warned that the Blues are infinitely more dangerous now.
He told Everton's website: “I think the Chelsea team from that moment is not the same Chelsea team as now. The players in the team are back scoring again, Diego Costa is scoring again, Hazard is gaining momentum so it’s going to be a totally different game.
"In football you should never compare one game to another. We will have to be better than them and sharper than them to win the game.
"It is not going to be easy but we are up for it, we have worked hard [to get to the quarter-final] and we are going to give our maximum."
Lukaku played just 15 times competitively for Chelsea after signing from Anderlecht for over £10 million in 2011, being loaned out first to West Bromwich Albion and then to Everton rather than given any serious first-team chances at Stamford Bridge.
The 22-year-old has netted 23 goals in 35 matches for Everton this season in all competitions, including scoring on each of his last three appearances.