The Spanish international is said to be aware he has no future at Manchester United under new manager Jose Mourinho.
Mouirnho sold Mata to Man United in January in 2014 during during his second spell at Chelsea and he still believes the attacking midfielder is too slow to be at the heart of the attacking element of his team.
He was not included in Vicente del Bosque’s Spain squad for Euro 2016 and last featured for his country in a friendly against England last November
The deal is said to be around £20 million, but there competition to land his signature and the price could increase if the club are able to start a bidding war.
Henrikh Mkhitaryan’s arrival at Old Trafford has signalled Mourinho’s intent over Mata’s future at Old Trafford even though he was a regular under Louis van Gaal.
Mata has previously played under Koeman during his time in charge of Valencia in 2007-2008. The side managed to win the Copa del Rey in 2008 and Mata played for four seasons before he joined Chelsea in 2011.
He featured in every single Premier League game for Man United last season and finished as the club’s third top-scorer across all competitions. He was vital to van Gaal’s side but may need to move to another Premier League club as he is unlikely to survive Mourinho’s overhaul at the club.