The German side ran out 4-0 winners at Parkhead, a result which leaves Ange Postecoglou's men bottom of their group after two defeats in two.
Celtic created plenty of good openings but came up against an inspired Lukáš Hrádecký in the Leverkusen goal making a number of excellent saves. At the other end of the pitch, however, it was a different story as the home side gifted the German side two first-half goals.
Bayer added two more after half-time despite a spirited performance at times.
So, how did the players involved rate out of ten.
Player ratings scale - 10: One of the best performances you've seen. 9: Outstanding. 8: Very good. 7: Good. 6: Average. 5: Poor, 4: Very poor. 3: A terrible day at the office. 2: Abysmal. 1: One of the worst performances you've seen.
9. Jota - 7
The Portuguese winger is positive, exciting and entertaining. Crated some good openings up against Jeremie Frimpong and went close to scoring. Photo: Alan Harvey - SNS Group
10. Liel Abada - 6
The winger drifted in and out the game. A couple of good crosses but missed a great chance to make it 2-1 on a rebound. Photo: Craig Williamson - SNS Group
11. Kyogo Furuhashi - 6
Gave Celtic much more threat through the middle. Great movement, some lovely runs but was constantly thwarted by Hradecky and put a good chance into the side netting. Photo: Ross MacDonald - SNS Group
12. James McCarthy - 5
Gave the ball away not long after coming on, in keeping with many of his team-mates. Photo: Ross MacDonald - SNS Group