At 15 years and 219 days old, Romero takes the record from Francisco Bao Rodriguez, who was better known as Sanson and was 15 years and 255 days old when he made his league debut with Celta Vigo in 1939.
Romero only began training with Mallorca's first team last month after spending the season with the club's youth teams. He has not even played for the B team yet.
A paperwork issue prevented him from making the squad in a June 13 defeat to Barcelona and he was an unused substitute against Villarreal and Leganes before finally making his debut against Madrid, coming on for the final five minutes.
"What you try to do is calm him down. He's a 15-year-old kid. The important thing is he's there because he deserves to be," Mallorca coach Vicente Moreno said after Wednesday's game. "He did well and he was aggressive. If we keep encouraging him, I'm sure he'll keep growing."
Romero only began training with Mallorca's first team last month after spending the season with the club's youth teams. He has not even played for the club's B team yet.
He was born in the Mexican city of Durango in 2004 but has Argentine parents. His father, Diego, was also a footballer and was playing for Alacranes at the time of his birth.
In the latter stages of his career, Diego played for Formentera on the Balearic Islands, which is where the family have since settled.
Luka started out playing for the academy at Formentera before moving to Mallorca. He has courted interest from Barcelona and Real Madrid in the past.
When he was just seven, he excelled in a trial with Barca but the Catalan club were unable to sign him because he was too young to live alone in La Masia at the time.
The player has already drawn comparisons with Lionel Messi from an early age. Not only is Romero from an Argentine family, but he is also a left-footed forward and has a similar playing style to the Barca captain.
Images of him training with Mallorca over the last month bare a striking resemblance to a young Messi, with his small frame, baby face and floppy hair.
"My whole family is Argentine and my dream is to wear the national team's colours," he said in an interview with the Argentine Football Association in 2018.