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.
1. Joe Hart - 7
The Englishman was good, producing very good stops in each half and had little chance with the goals. Photo: Ross MacDonald - SNS Group
2. Anthony Ralston - 4
A difficult evening for the full-back. Started strongly with some wholehearted defending but struggled as an inverted full-back, losing possession for the second goal. Photo: Ross MacDonald - SNS Group
3. Cameron Carter-Vickers - 6
The big centre-back was pretty solid throughout. Gets in good positions. Penalty decision given against him was harsh. Photo: Craig Foy - SNS Group
4. Carl Starfelt - 5
The Swede was decent in one v one situations and nearly scored but was part of a defence which was too exposed. Photo: Ross MacDonald - SNS Group