Victor Wanyama claimed that the Celtic fans stripped the players of their confidence after St Johnstone took the lead in their Scottish Premier League win at Parkhead on Sunday.
The Kenya international, 20, signed from Belgium club Germinal Beerschot in the summer, made his competitive debut for the Hoops in defence, in what turned out to be a baptism of fire.
The Celtic faithful shrugged off Kris Commons' third-minute penalty miss, but grew increasingly agitated as Neil Lennon's men squandered several chances thereafter, despite not being at their best.
Wanyama was aware of his new team-mates' confidence disintegrating as the second half wore on and St Johnstone held firm.
"It was very disappointing to lose on my debut," he said.
"When they scored the fans wanted us to play better, and that's when the confidence went down.
"The crowd wanted more but we couldn't handle it, so we need to work on that.
"The manager was disappointed after the game, and that is normal because we didn't do that well.
"We played okay but we didn't take our chances, that was the problem.
"At home we can beat anybody, but we were unlucky."
The Kenyan believes that Celtic can recover their form to make it through to the Europa Leauge group stages.
"I think we will get a good result," he said.
"The first game was goalless, so no one has an advantage and the chance is still open.
"We can go there and win."