The Frenchman is undergoing a medical ahead of a move to Schalke 04.
According to Fabrizio Romano, a deal has been agreed between the club and Jullien will join the German giants on loan.
Jullien, signed from Toulouse for £7million in 2019, featured just once under Ange Postecoglou last season following his return from a long-term injury which kept him out for more than a year.
The 29-year-old was understood to be keen to fight for his place at Celtic but interest emerged from Germany as well as clubs in the French top-flight.
Last season, he was behind Cameron Carter-Vickers, Carl Starfelt and Stephen Welsh in the pecking order.
Jullien will depart having played 62 games for Celtic, playing a pivotal role in helping the club win a treble in 2020.
Meanwhile, Celtic are closing in on the signing of Alexandro Bernanbei from Lanus.