Injuries restricted Mattia Caldara's game time at Milan and he has completed a move back to Atalanta.
Mattia Caldara has completed a return to Atalanta from Milan on an initial 18-month loan deal.
Atalanta also have the option to make the move permanent, with reports suggesting the Rossoneri would be due a €15million fee to do so.
Much was expected of Atalanta academy product Caldara when he moved to San Siro in a €35million move from Juventus, which saw Leonardo Bonucci transferred back to the Bianconeri in a similar deal in August 2018.
However, the centre-back has made just two competitive appearances for the Rossoneri, largely due to two serious injuries.
Caldara sustained an Achilles tendon rupture in September 2018, before rupturing the anterior cruciate ligament in his left knee in April.
A statement from Milan read: "AC Milan announces the agreement for the player Mattia Caldara to join Atalanta on an 18-month loan deal with right to buy.
"AC Milan wishes Mattia the best of luck in his future sporting career."
Atalanta, who defeated Milan 5-0 last month, drew 1-1 with the Rossoneri's city rivals Inter on Saturday and are fifth in the Serie A table.
Milan are eighth after a 2-0 win at Cagliari.