Neil Lennon's side took the lead in the Camp Nou and it took a 94th minute goal from Jordi Alba to give the Spanish giants all three points. However, despite the gut-wrenching nature of the defeat, Israel international Kayal says the squad still believe that qualification can be achieved.
"We have always believed we can do something at this level. We have two home games to come as well as having to go to Lisbon," Kayal told the Daily Record.
"Of course we believe it's possible to qualify.
"We thought we could do it when the draw was made and we believe it now."
Having gained an impressive victory in Moscow last month, Celtic host Barcelona before an away trip to Benfica and the final group game against Spartak Moscow. They currently sit second in Group G on four points.