The 39-year-old looked far from his best when he reacted slowly to get down to the first goal from Bilal El Khannouss double and also looked like a keeper ready for retirement when he failed to keep out a low drive from Jamie Leweling, which made it 3-1.
The Denmark international was subsequently booed by a section of supporters every time he touched the ball in the next five minutes, and the crowd cheered sarcastically when he got down to save a low drive from Tiago Tomas.
Coach O’Neill, after the match, admitted that his team had conceded some “really poor goals”: “We played Feyenoord in a game that we probably had to win, and I think it was 1-1 at the time, and he made an unbelievable save. Without that, we probably wouldn’t be here.
“This is a team game, and we all have to deal with it.”
O’Neill was also asked if he might consider playing Viljami Sinisalo in the wake of the frustation which the Celtic fans have shown towards the Danish keeper and said: “You’re asking me a question here on Thursday night. Who knows what might happen at the end of it all?
“All I will say is Kasper Schmeichel has played so many games, he’s had some dips in his time, and he’s come right back.
“Kasper Schmeichel has done exceptionally well for me in my time here.”
O’Neill criticized the fact that fans disrupted the start of the game by throwing dozens of mini balls onto the pitch seconds after kick-off in a protest against the board.
O’Neill said: “Anybody who thinks that’s a good idea needs their head examining.
“Can you imagine being a Stuttgart player now, and you’re worried about the crowd? The atmosphere, to begin with, was fantastic. And then suddenly that happens.
“One, it disrupts the game, obviously. And secondly, if I’m a Stuttgart player, I’m thinking, ‘I don’t feel too bad here. Don’t feel too bad, instead be intimidated by the situation. That helps no one."