The 26-year-old returned to France where he has had surgery on the injury picked up at training on Monday.
Lennon said: "It was an innocuous thing. He was running for the ball and we heard this pop.
"We thought he had clipped someone's ankle but obviously straight away he knew something was wrong and he went over to the side.
"He had a complete rupture of his Achilles tendon, he has had surgery in France. We are waiting on how well the operation went but we are looking at maybe six months, so we are hoping to see him sometime in the New Year."
Mouyokolo has made two starts since signing for the Scottish champions in July, following his comeback from a cruciate ligament injury.
He was a free agent after his contract with Wolves was terminated by mutual consent in January and was handed a one-year deal by the Parkhead club after impressing on trial during their pre-season trip to Germany and Austria.
Lennon said: "It is devastating for him. He has worked so hard.
"This is a totally separate injury to the previous ones he had. It can happen to anyone at any time.
"He has just had horrendous luck and he will support him all the way and hopefully we can get him back up and running February or March."
The Northern Irishman continued: "I went in to see him and he was obviously really upset.
"He had given up his contract at Wolves to find a new club and came on trial with us and impressed us.
"It is a devastating blow for him because he is still a young guy, he is 26, he's had long bouts of absence with injury and we were all hoping that he would get a clean run but obviously that is not the case but, as I said, it is totally incidental to the previous injuries he has had.
"He wants to come back here as quickly as possible so we are hoping to see him the back end of next week."
The Parkhead club travel to Hearts in the Scottish Premiership on Saturday before they begin their Champions League campaign with a trip to the San Siro to play AC Milan next Wednesday night.
Lennon played down the effect the defender's injury will have on his squad over the next months.
He said: "I still have decent cover there.
"Steven was low-risk in terms of remuneration.
"We brought him basically as a squad player for SPFL games and maybe, if he showed a bit of form, for Europe.
"We are very pleased with his progress and it is a blow [but] we have decent cover. We are not stretched as of yet."