Later on Monday, Lukaku's brace helped Inter to a win over Napoli, taking them atop the standings on goal difference from Juventus.
Cristiano Ronaldo scored a second-half hat-trick to lead Serie A champions Juventus to a 4-0 win at home to Cagliari on Monday, taking his tally to eight goals in his last five league games.
The Portuguese pounced on a mistake in the Cagliari defence to break the deadlock four minutes after halftime before adding the second from a penalty as he scored for the fifth league game in the row.
After setting up Gonzalo Higuain for the third in the 81st minute, he completed the rout by finishing off a counter-attack one minute later.
Ronaldo, who completed the 36th league hat-trick of his career, two more than Lionel Messi, took his tally this season to 15 goals, including 13 in Serie A.
Sixth-placed Cagliari defended stubbornly in the first half, restricting Juventus to one real chance when defender Merih Demiral’s header thumped against the crossbar from a corner.
But it began to unravel for the Sardinians when Ragnar Klavan’s pass across the face of the Cagliari area was intercepted by Ronaldo who glided past goalkeeper Robin Olsen and scored into the empty net.
Giovanni Simeone headed against the crossbar for Cagliari but they were undone by more careless defending when Marko Rog tripped Paulo Dybala as he weaved his way into the area and Ronaldo sent Olsen the wrong way from the spot.
With nine minutes left, Ronaldo sent substitute Higuain free down the left and the Argentine cut inside before scoring with a deflected shot.
One minute later, Douglas Costa burst forward and slipped the ball to an unmarked Ronaldo who placed his shot between Olsen and the near post.
Cagliari then hit the woodwork for a second time as Joao Pedro struck the post.
Ibrahimovic return fails to ignite Milan in 0-0 draw
Zlatan Ibrahimovic's return failed to inspire AC Milan as the fallen giants were held to a 0-0 draw at home by Sampdoria in Serie A on Monday, failing to score for the third league game in a row.
The charismatic 38-year-old Swede was brought on to a rousing reception from the San Siro crowd in the 55th minute but Milan could still not find a way past their lowly opponents.
In his previous stint at the club, Ibrahimovic helped Milan win Serie A in 2010-11, their last major trophy. He left for Paris St Germain at the end of the following season.
Milan also drew their previous home game 0-0, against Sassuolo, and Monday's result left them a miserable 12th in the Serie A standings, below Hellas Verona on goal difference.
In between, Milan were thrashed 5-0 by Atalanta, who on Monday beat Parma by the same scoreline with Josip Ilicic scoring twice to close in on the top four.
Lukaku double gives Inter impressive win at Napoli
Inter Milan forward Romelu Lukaku scored twice, the first from a run which started inside his own half, to lead his side to an entertaining 3-1 win at Napoli which kept them top of Serie A on Monday.
Lautaro Martinez was also on target as Inter claimed their first away win against Napoli since 1997.
Inter lead the standings on goal difference from Juventus, who beat Cagliari 4-0 earlier on Monday, with both teams on 45 points. The head-to-head record will be used once the two sides have met twice.
Napoli, on the other hand, suffered their second defeat in three games since Gennaro Gattuso replaced Carlo Ancelotti as coach and stayed eighth with 24 points despite showing flashes of their old style.
"This win is worth a lot," said Inter coach Antonio Conte. "It's never easy to win at Napoli for anyone. We faced the match with great personality and this will give us more self-confidence and belief. We need to carry on like this."
Napoli also had chances before Lukaku put Inter ahead after 13 minutes. The Belgian, who now has 14 league goals this term, intercepted a pass just inside his own half, charged forward and scored with a shot which went in off the post.
His second in the 33rd minute came with a helping hand from Napoli goalkeeper Alex Meret, who seemed to have Lukaku's shot covered but allowed it to slip through his legs into the net.
It was tough luck on Meret who had previously produced two good saves to keep Inter down to one goal.
Napoli kept plugging away and Arkadiusz Milik tapped in Jose Callejon's low cross to pull one back in the 39th minute. The Pole should then have equalised when he had a free header just before halftime but directed his effort well wide of the goal.
Napoli continued to press after the break but, despite some flowing moves, they failed to find the final pass and Inter went further ahead just after the hour.
Matias Vecino floated over a cross from the right which Napoli defender Kostas Manolas managed to intercept but, in doing so, laid the ball off for Martinez to score his ninth league goal of the season.
"We made decisive mistakes in all three of the goals we conceded," said Gattuso.
"However, we showed we can play football and we need to continue doing that. We have to improve in various areas of the pitch."
Edited by Laxmi Negi