The Hoops conceded a goal to Alessandro Matri in the third minute of the Parkhead clash before the Italian giants netted twice late through Claudio Marchisio and Mirko Vucinic and Lennon was rueful of the sloppy nature in which his side were beaten in the Round of 16 first leg.
"We need a miracle," he told ITV1. "But it's the harsh reality of Champions League football and some of my young players will learn a huge lesson tonight.
"I thought for 70-odd minutes, until the second goal, we were by far the better side - but you can't give away goals like we gave away, sloppy."
Celtic will now go into the return leg in Turin on March 6 with a mountain to climb as they face elimination in the first knockout phase of Europe's premier club competition.