The on-loan teenager took his season's tally to 17 goals as Sir Alex Ferguson ended his Manchester United reign with an amazing 10-goal thriller - the first 5-5 game in Premier League history.
United led 3-0 and 5-2, only for the Baggies, who had been dismal for most of the first half, to fight back with a Lukaku treble and goals from James Morrison and Youssouf Mulumbu.
Shinji Kagawa, Alexander Buttner, Robin van Persie and Javier Hernandez scored for United to add to a Jonas Olsson own-goal.
Albion started brightly enough and yet found themselves 2-0 down inside nine minutes thanks to some dismal defending.
First, Javier Hernandez chased a seemingly over-hit Tom Cleverley pass into the right-hand corner and crossed to the near post, where Kagawa was in acres of space to head home.
And from then on the visitors took complete control of the game, with the second goal arriving in double-quick time.
The Baggies gave Antonio Valencia time and space to burst down the right and cross for Olsson to put through his own net and make it 2-0.
It should have been 3-0 when United captain Michael Carrick crossed from the right and Hernandez misdirected a header badly.
But the third goal arrived on the half-hour as Albion again invited United to shoot and the Premier League champions took full advantage.
Hernandez and Cleverley combined well on the edge of the box to feed Buttner, who surged forward into space from left-back and received all the time he needed to drill home a low, left-footed shot.
Albion had a rare chance when Chris Brunt found some space on the left and crossed to Shane Long, who sent a far-post header over.
But United should have had a fourth goal when more dreadful defending from the hosts allowed Valencia to pull a raking cross back in front of goal for Hernandez, who was unmarked but prodded a tame effort wide of the far post.
Albion gave their fans some hope five minutes from half-time when they pulled a goal back.
Youssouf Mulumbu seized on a loose pass in midfield and fed Graham Dorrans, who crossed for Morrison to steal ahead of Phil Jones and turn the ball home.
But even then there was time for another scare as Hernandez was allowed to shoot, Ben Foster saved but the ball hit Gareth McAuley, forcing the Baggies keeper into an acrobatic save to prevent a comical own-goal.
Albion raised hopes further five minutes after the restart when half-time substitute Romelu Lukaku reduced United's lead to one goal.
He cut in from the left and let fly with a right-footed shot that bent in off the outstretched left hand of Anders Lindegaard.
But United restored their two-goal cushion just three minutes later with another move down the Baggies' left flank, where Brunt was operating as a makeshift left-back following a half-time reshuffle.
He was beaten by Valencia, who crossed for Van Persie to get ahead of Olsson and prod the ball home.
And they extended the advantage on 63 minutes when Hernandez grabbed his second.
Again it was poor defending from the hosts as Shinji Kagawa fed substitute Ryan Giggs, whose cross was turned in by the Mexican.
In a crazy game, Albion should have made it 5-3 when a Dorrans corner found Olsson at the far post but he lifted a shot over when it looked easier to score.
And, in a ridiculously open game, Albion twice went close again in the moments that followed with Lukaku and Dorrans both firing just off target.
It took another amazing twist with two goals in a minute for the Baggies to make it 5-4.
First substitute Markus Rosenberg split the United defence and Lukaku rounded Lindegaard to score.
And then Billy Jones pulled a cross back for Mulumbu to slide home a shot with nine minutes to play.
The remarkable comeback was complete with four minutes to play.
Dorrans' cross from the left was bundled back across goal by Mulumbu for Lukaku to force it home.