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After training with Mallorca's senior team for the first time on June 1, Romero, who plays as an attacking midfielder, came on as a substitute in the 83rd minute of his team's 2-0 loss at Real Madrid.
"It was an unforgettable moment," Romero wrote on Twitter after the game.
"Thank you to everyone on the technical staff and at Real Mallorca for giving me this opportunity. I will never forget this day. It was a shame to lose but we keep believing."
Romero beat the record held for 81 years by Francisco Bao Rodriguez, known as 'Sanson', who played on December 31, 1939 aged 15 and 255 days for Celta Vigo against Sevilla.
Mallorca coach Vicente Moreno was seen talking to Romero before he brought the youngster on as a substitute.
"I tried to just keep him calm," Moreno said.
