Celtic paid a considerable fee to sign Jullien from Toulouse, but he is behind in terms of his fitness.
The powerful defender is yet to make his debut for Celtic, and Lennon admits that Jullien still needs to work on the training ground before he is ready to play.
Lennon said: “Christopher Jullien isn't up to speed yet and we are still looking to strengthen there too.”
If Celtic do bring in another centre-back then their options in the position could be strong.
Celtic have lost Dedryck Boyata and Filip Benkovic, but if they sign Jullien and another centre-back then they should be well stocked in the position.
Celtic got off to a winning start this campaign, as they beat FK Sarajevo 3-1 in Champions League qualifying.
It remains to be seen whether Jullien will be considered sharp enough to feature in the second leg for the Bhoys.
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