The Frenchman has been out of action for nearly 13 months after rupturing his cruciate ligament in a collision with the post in a match against Dundee United in December 2020.
The 28-year-old - a £7m signing from Toulouse in the summer of 2019 - returned to training recently and took to social media to tell fans, "2022 will be a brighter year for me", along with the hashtag "#BackinBusiness".
And the centre-back has now edged closer to making a first team comeback after featuring in a bounce match at Lennoxtown.
According to the Scottish Sun, he was part of a strong Hoops squad which beat St Mirren 1-0 in a closed doors game at the club’s training base.
New Japanese signings Reo Hatate, Yosuke Ideguchi and Daizen Maeda also made their first appearance with Owen Moffat on the scoresheet.
The return of Jullien will be "like a new signing" according to Celtic head coach Ange Postecoglou but he is not expected to feature against Hibs in the first match after the Premiership winter break on Monday.