The 27-year-old only joined the 'Saints' in January from French club Monaco, but largely failed to impress on the South Coast.
The Argentine failed to score in his first seven games for the club and then did not feature in Southampton's last seven games of the season as they narrowly staved off relegation.
Early on Sunday, Southampton confirmed that Carrillo had joined Legnaes on loan, where he will link up with former Southampton manager Mauricio Pellegrino, the man who brought him to St Mary's.
'Lega' posted a photo of the striker arriving at Madrid's Barajas airport and posing with a club scarf to their Twitter account, saying that he would join his new team-mate on Monday for pre-season training and would be presented to the press 'shortly'.
8 July 2018