Stokes has been mired in controversy throughout his time with the Hoops, but Lennon believes the player has his head screwed on after spending a long period on the injury sidelines last season.
And Lennon has called on Stokes to rise to the challenge of filling the void that has been left with the departure of Gary Hooper to Norwich this summer.
The Celtic boss revealed Stokes' ill-discipline and off-field incidents meant there was a stage where he was "walking on very thin ice" at the club, but he expects to see a more mature player this season.
"I had him in the office a couple of times," said Lennon. "He was walking on very thin ice. He was hurting himself.
"It was nothing to do with stuff last year, the stuff in Dublin, it was just his indiscipline around the training ground and the team. So he's really tidied that up now and become a really good professional.
"He should be looking to be the No.1 striker here, to continue scoring goals at domestic and European level and he should be wanting to get in the Irish team.
"Again, he let himself down there. Unfortunately, Trapattoni has gone now but you are hoping that when a new manager comes in the slate is wiped clean for him and he can make a charge on that aspect of his game as well."
Lennon's renewed faith in Stokes was perhaps best reflected in the fact he started the Irishman in the San Siro on Wednesday night - the forward has been repeatedly overlooked for the big occasion in the past, regardless of how well he has performed in domestic games in the lead up.
"I had question marks over his temperament in big games," said Lennon. "I think his athleticism has improved now, his fitness is a lot better.
"His physical strength is a lot better than it was, and certainly his football. He was always a really good player, his striking movement was always good. Very adept with both feet.
"But he was curtailing his own progress by his own indiscretions and indiscipline. But I think now, certainly since he returned from his injury, he has really knuckled down and he's doing his talent justice now.
"He's always had goals in him and he will score goals at Champions League level if we get the right service to him.
"As a lone striker you sometimes have to create your own opportunities, but the one he had in the first half in Milan, he's latched on to a mistake and I fancied him to score, but he just sclaffed the shot a little bit.
"And if his free-kick had been an inch or two lower we'd all be raving about him.
"There were things he was doing in the game on Wednesday, he was challenging for balls in the air whereas before he was a little bit wishy-washy. He's looking to be the No.1 here and he's enjoying that challenge."