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The defender made 38 total appearances for the club during his loan spell, scoring three goals, with two of those coming in La Liga and one in the Europa League.
With his future at Real Madrid uncertain, Reguilon had been linked with either a permanent transfer abroad or a loan back to Sevilla, which would have allowed him another season to prove he is ready for a regular spot at the Bernabeu.
However, sporting director Monchi conceded last week his side would likely miss out on the 23-year-old, who is also a reported target for Manchester United as well as Spurs.
The Spaniard has been heavily linked with the Red Devils throughout the summer, with Manchester United legend Rio Ferdinand calling for Reguilon, two forwards and a new centre-half to be snapped up before the window closes.
It appears that may be a possibility as, on Monday, Sevilla confirmed the arrival of Argentina international Acuna on a four-year deal for a reported upfront fee of €10 million (£ 9m/$12m), all but confirming that Reguilon would not return to the club.
Acuna spent three seasons with Sporting, making 134 appearances for the Portuguese giants in all competitions, providing nine goals and 23 assists for the club.
The defender originally joined Sporting from Racing in Argentina, where he starred from 2014-17 before making the move to Portugal.
He becomes Sevilla's fifth signing of the window following deals for Ivan Rakitic, Oscar Rodriguez, Yassine Bounou and Suso, the latter two making their loan moves permanent.