The Northern Irishman left Parkhead after winning three league titles and two Scottish Cups in four years, to be replaced by former Stromsgodset boss Ronny Deila, whose honeymoon period ended abruptly in the Pepsi Arena.
With group stage aspirations up in the air, the spotlight has fallen on the strength of the Celtic squad.
It has been augmented so far by former Hearts and Sunderland keeper Craig Gordon, who was out of contract and is currently third choice, and Norwegian forward Jo Inge Berget, who was signed on a six-month loan deal from Cardiff City and who made his debut against Legia.
Speaking to STV, Lennon addressed the issue of investment at the Parkhead club in regard to his decision to leave.
"I had my own reasons for leaving and they were varying," said the former Hoops captain.
"I had four great years and 14 great years as a player, coach and manager. It was time for me to move on personally, the club understood that and I had great people in the background working with me.
"It wasn't the sole reason but it may have been one of the reasons."
Speaking to BBC Radio 5 Live earlier in the day, Lennon discussed his successor's plight following the heavy defeat by Legia, which could have been worse had the home side not missed two penalties.
"I wouldn't say it was inevitable but I think it might have been on the cards," he said.
"There has been no investment at the minute in the team and Ronny hasn't had much time to bed himself in with (assistant manager) John Collins and get to know the players.
"Those qualifiers are always really tricky. It took me a year or two and I remember Gordon Strachan in his first year (as manager) losing 5-0 in Bratislava, he won the second leg 4-0 but it was too big a gap to recover.
"So it can happen.
"They are not out of the tie but they are going to have to play supremely well to get through now."
The night had started well for Celtic when midfielder Callum McGregor put the visitors ahead after seven minutes but a double from Miroslav Radovic and the dismissal of Parkhead defender Efe Ambrose just before the break turned the match firmly in Legia's favour.
In a remarkable second-half captain Ivica Vrdoljak spurned two penalties conceded by Hoops skipper Charlie Mulgrew - missing the target with the first with Fraser Forster saving the other - but Michal Zyro and Jakub Kosecki added further goals to heap pressure on the Hoops ahead of next week.
If Deila's side fail to overturn their first-leg deficit they will drop into the play-off round for the Europa League.
While believing that Celtic can still rescue the tie, Lennon insists it will not be disastrous if the Scottish champions miss out on European football's elite club competition this season.
"Champions League football is always important," he said. "It gives the fans and players something to look forward to as well but the club is financially healthy.
"They can maybe take the hit if they don't make the Champions League this year.
"I don't think it will be disastrous for them in the long run."