The Spain international's arrival gives Real Madrid four senior options in the role, with Cucurella now joining Ferland Mendy, Alvaro Carreras and Fran Garcia in direct competition for a single starting spot.
According to Marca, Cucurella's move was completed for a fee in the region of €55 million plus add-ons, with the 27-year-old signing a six-year contract running until June 2032.
Cucurella arrives with the look of an immediate starter. He has established himself as one of the most consistent left-backs in the Premier League over the past four seasons and remains a fixture for Spain at the World Cup, where he is part of Luis de la Fuente's squad in North America.
His signing responds directly to a problem that has dogged Real Madrid for several seasons, with the left-back position repeatedly undermined by injuries and inconsistent form. New head coach Jose Mourinho is expected to start the season with Cucurella as his first-choice option.
The most delicate situation belongs to Ferland Mendy.
The French defender underwent surgery in May on a rectus femoris injury in his right leg, with a recovery timeline of around five months that will rule him out of a significant part of the season's opening months.
The injury draws renewed attention to a pattern of physical setbacks that has repeatedly disrupted Mendy's career at the Santiago Bernabeu. Despite his contract still being active, his footballing situation is now complicated both by a lengthy recovery and by the arrival of fresh competition in an already saturated position.
Alvaro Carreras also emerges weaker from the move.
The Spanish defender arrived last summer as an important bet on the long-term future of the left-back role, but as the months passed that scenario cooled considerably. Although Carreras featured throughout the season, he never fully established himself as an undisputed alternative and lost standing within the rotation.
Cucurella's arrival represents an additional obstacle for a player who, barely a year ago, looked destined to lead the position for the next decade.
That leaves Fran Garcia as the player most likely to depart. The Spanish full-back ended last season in better form than Carreras, even finishing the campaign above him in the pecking order, partly by taking advantage of Mendy's injuries to gain more playing time.
Even so, his future appears the most exposed. It already did before Cucurella's signing, but the new arrival increases the likelihood of an exit. Garcia came close to leaving for Bournemouth in the January window, with the English club tabling an offer that Real Madrid rejected at the time, primarily due to a lack of defensive cover.
His contract situation, market value and the interest he attracts elsewhere make him the most straightforward piece to move in order to balance the squad. His departure now looks like the most logical operation to ease a position that has shifted, within the space of days, from a genuine concern into a problem of surplus options.
Real Madrid now find themselves with a newly signed starter, a long-term injury case, a once-promising project who has lost momentum, and a player with market value who will struggle to find minutes. At Valdebebas, the message is clear: sooner or later, the left-back position needs to be thinned out.