The left-back was promoted to the first team by the former Spain coach and started for Los Blancos in the 1-0 loss against CSKA Moscow in the Champions League.
Lopetegui was sacked shortly afterwards, but the partnership would prove pivotal later on at Sevilla.
Before that, Santiago Solari made Reguilon his first-choice left-back at Madrid ahead of Marcelo and the youngster was one of the team’s standout players in an otherwise disappointing season.
“Reguilon is an example to all the players in the youth system who want to make it to the first team,” Solari said. “With talent and dedication, you can go very far.”
Back-to-back Clasico defeats against Barcelona and a Champions League elimination against Ajax later in the same week saw Solari sacked later in the season, but Reguilon was perhaps his greatest legacy.
And by that time, the left-back had caught the attention of his new coach at Tottenham, Jose Mourinho, who was working as a pundit following his sacking by Manchester United.
“One of the players that could show that Real Madrid traditional desire to compete against Barcelona was Reguilon,” the Portuguese said in his role as a television analyst for beIN SPORTS after one of the Clasico clashes.
When Solari left, Zinedine Zidane returned for a second spell and Reguilon drifted out of the first-team picture, starting just two of the remaining 11 games.
Zidane brought back Marcelo from the cold and then signed Ferland Mendy in the summer. There was no place for another left-back.
Many Madrid fans were unhappy at the decision, but their loss was Sevilla’s gain as Monchi and Lopetegui took Reguilon to the Sanchez Pizjuan.
Like the team itself, Reguilon grew in stature on the left side in what turned out to be a spectacular season for Sevilla.
Lopetegui’s side finished fourth in LaLiga, level on points with third-placed Atletico Madrid, and also won the Europa League.
Reguilon was key to their success. The 23-year-old loves to carry the ball forward and is a very skillful dribbler always looking to exploit spaces and to get into attacking positions.
With good pace and athleticism, Reguilon can press high up the pitch, gets up and down to cover his flank well and is also a tenacious tackler.
In 2019-20, his forward runs were an essential element of Sevilla’s attacking armoury and he finished the season with three goals and five assists in 38 appearances. Only Mendy, the player who took his place at Madrid, had a superior season at left-back in LaLiga.
His final ball can be better, along with his defending on occasions, but the commitment and desire to improve are evident for all to see and Sevilla would love to have taken him back for a second season.
The Andalusians were priced out of a move for the 23-year-old, but Lopetegui recently said: “He has grown a lot this year and he is a better player now than when he arrived.”
He is also a full Spain international after making his international debut under Luis Enrique against Ukraine earlier this month and should go on improving.
“I think he’s a player who is for the present, but also a player who needs development and consistency in his evolution,” former Tottenham manager Mauricio Pochettino said on Thursday.
“They need to work closely with him. He has great potential, but he’s a young player who has to keep growing.”
He should. And if he does, he can become one of the very best full-backs in the world.