The 25-year-old netted twice in Madrid’s 3-2 win over Villarreal in May in what was only his third league start of the campaign - whilst he is thought to be a peripheral figure again next season.
He has not featured in any of the club’s three pre-season friendlies to date while Tottenham have also been linked with a move for the striker who spent the 2017/18 campaign in Ligue 1 with Lyon.
The Dominican forward had previously netted just two goals across all competitions with game-time limited and suffering a series of hamstring injuries.
Milan are said to have asked about the striker’s availability during talks to sign Theo Hernandez from Madrid in a €20m deal but need to first offload Patrick Cutrone.
Mariano was said to have been keen on a move to the Mestalla should Madrid dispense with his services, due to their participation in the Champions League.