The Scottish champions fell to a 1-0 loss against the relegation-threatened team as their miserable title defence suffered another setback and Lennon could only say sorry to the supporters.
The Celtic manager said: "I'm sorry [to the fans]. We've let them down again.
"It's not for the want of trying to win a game of football. So we're bitterly disappointed with that."
Celtic - who now sit 18 points behind rivals Rangers at the top of the league - missed a number of chances in the loss and Lennon feels that the result tells the story of their campaign.
He told Sky Sports: "It’s a strange one. I think it sort of encapsulates the season really.
"We’ve dominated the game, we’ve missed some unbelievable chances and then we can’t defend a set play for some reason. After that, we missed another great couple of chances.
"It’s a head-scratcher tonight for sure, we were totally dominant.
"We came up here and won 5-0 and we didn’t play anywhere near as well as we did tonight, and we got nothing for it.
"It’s a difficult one to stomach at the moment."
Lennon was also angered by his team's inability to defend set-pieces.
He said: "I can't do it for them. It wasn't a problem for us last season but it's been a real problem this season with more or less the same personnel.
"You have got to want to go and head the ball, block runners. I don't see us getting any free headers in the opposition box but I have seen that plenty of times for us and it's not been good enough."