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The 21-year-old, who came through the youth system at Marseille, has signed a five-year deal with Milan.
Hernandez spent last season on loan at Real Sociedad, scoring one goal in 28 appearances.
He only played 23 competitive games for the Real first team, and it was no surprise to see him leave after coach Zinedine Zidane signed his fellow French left-back Ferland Mendy from Lyon for around 50 million euros.
Milan are in a period of transition after coach Gennaro Gattuso left the club, with Marco Giampaolo taking over and Paolo Maldini and Zvonomir Boban arriving as directors.
The seven-time European champions narrowly missed out on the Serie A top four last season to fail in their bid to secure a first Champions League campaign since 2013-14.
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