Victor Wanyama claimed Celtic fans stripped the players of their confidence after St Johnstone took the lead in their shock Clydesdale Bank Premier League win at Parkhead yesterday.
The Kenya international, 20, signed from Belgium club Germinal Beerschot in the summer, made his competitive debut for the Hoops in defence alongside Daniel Majstorovic in what turned out to be a baptism of fire.
Dave Mackay's second-half strike gave the visitors their first win in the east end of Glasgow since 1998 to prevent the home side going top of the table.
The Celtic supporters shrugged off Kris Commons' failure to convert a penalty in the third minute 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 of some of us 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 only problem.
"At home we can beat anybody but we were unlucky."
Saints keeper Peter Enckelman saved Commons' poorly-struck spot-kick to set the tone for the rest of Celtic's frustrating afternoon which saw Commons and substitute Paddy McCourt both hit woodwork.
Lennon berated his players after the match and will have to work hard to rebuild confidence ahead of the second leg of their Europa League play-off against Sion in Switzerland on Thursday night, which is still very much in the balance following the goalless draw at Celtic Park.
Defender Charlie Mulgrew is surely doubtful after coming off at half-time against the Perth men with a hamstring injury.
Consequently, Wanyama may keep his place if Kelvin Wilson fails to recover from his leg injury, although Lennon is confident that the former Nottingham Forest stopper, along with Beram Kayal and Gary Hooper, will be back fit.
The Kenyan believes that Celtic can recover their form to make it through to the 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."