The French defender departed the Parkhead club in the summer, making a £1m transfer back to his homeland with Ligue 1 side Montpellier, having cost Celtic seven times that amount when making the move from Toulouse three years ago.
Jullien's first season in Scotland proved a hugely successful one as he established himself as a first-team regular under Neil Lennon and played a significant role in the club claiming all three domestic trophies to complete an unprecedented quadruple treble.
However, his second season in Paradise turned into a nightmare as he suffered a cruciate ligament injury in a collision with the post during a match against Dundee United in December 2020 which would keep him out of action for 18 months.
Despite returning to fitness last season following a long road to recovery, Jullien only made one substitute appearance under Postecoglou, coming off the bench for the last 15 minutes of a 4-0 win over Raith in the Scottish Cup.
With Cameron Carter-Vickers, Carl Starfelt, Stephen Welsh and Mortiz Jenz all ahead of him in the pecking order for a starting berth in defence, a parting of ways during the summer was inevitable with Jullien now back to being a first-team regular in the French top flight.
Despite his lack of opportunities under Postecoglou, the 29-year-old does not appear to be holding any grudges as he praised his former boss for helping develop his game by making him more "tactically aware".
Speaking to the Ligue 1 website, Jullien said: "My first season was amazing! We managed to win the three national trophies.
"But the second was not as prolific since we only won one trophy. So yes, the list has grown. I don't regret at any time having signed there. I had an incredible experience!
"When you don't play for a year, you learn a lot about yourself, your surroundings and your family. After such an injury, you can only become stronger mentally.
"But the thing that struck me the most is that when you're abroad and you're not playing, your name comes out of the radar very quickly. Many people asked me what I was up to and whether I played or not.
"It a very long absence from a match, because I really came back from my injury in November 2021. The problem I encountered on my return was that the team was doing very well and the coach didn't need to integrate me into the eleven.
"Even though I didn't play last season and it was complicated on a human level with coach Ange Postecoglou, I learned a lot. He is a coach who relies a lot on video and who tries to get into the opponent's head.
"Thanks to this, I realised that when you push the opposing team to do exactly what they don't want, you can outwit them to an unimaginable extent. It's like playing chess. If the opposing team can't find a parry, you're in for a treat.
"That's why I think I've become more tactically savvy. But I always try to improve on this aspect."