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.
5. Adam Montgomery - 5
The youngster had a trying evening. Like Ralston struggled to play that inverted full-back role and the responsibility which comes with it. A key learning experience. Photo: Ross MacDonald - SNS Group
6. Callum McGregor - 6
A massive boost at the base of the midfield. Gives Celtic greater balance but at times overrun. Photo: Craig Williamson - SNS Group
7. Tom Rogic - 6
Dangerous in stages with some probing midfield play and looking to link with Kyogo. Can be a man down when the ball goes goal side and team attack Celtic with pace. Photo: Ross MacDonald - SNS Group
8. David Turnbull - 5
An off night for the midfielder summed up by the error for the first goal. Got caught in two minds and slipped. Wasn't influential in possession. Photo: Ross MacDonald - SNS Group