The gutted Hoops boss lambasted his players as poor imitations of the team he has carefully rebuilt on his watch. He said: "That was an impersonation of my players."
The shock defeat to County came just a few weeks after he took over as interim-manager in the wake of Tony Mowbray's departure.
But Lennon feels the pain of yesterday's defeat more because this is now HIS team. He has put his faith in the squad he has painstakingly crafted, and led them back to the top of the game in Scotland, and secured a last-16 place in the Champions League.
But yesterday he watched as his side's dreams of a treble crashed against a St Mirren side who fully deserved their 3-2 win.
Asked if this defeat hurt him more than the one against Ross County, Lennon responded: "Yes."
He added: "I don't know who that was out there yesterday. They certainly didn't have any of the attributes which we have tried to instil into our team in the time I have been in charge."
Saints caught Celtic cold, despite all the warnings which Lennon had sounded ahead of heading to Hampden, where their record with him in charge now reads played nine, lost five, won four.
Goals from Esmael Goncalves, Paul McGowan and Steven Thompson were answered by an equaliser just before the break from Gary Hooper then a consolation strike in the final seconds from Charlie Mulgrew.
The defender had seen his spot-kick saved just three minutes after half-time, and Lennon reckons that was when the comeback came to a shuddering halt.
He said: "The heads seemed to drop, and that's not like us. Our discipline and our shape was poor. We had too many individuals, and did not have a team performance."
Lennon tore into his players in the dressing room after the game, and revealed his was the only voice heard.
Ater having the night to reflect, the manager is not sure his players will turn up at Lennoxtown today ready to admit to their failings.
He said: "They had the chance to do that in the dressing room after the game, and it was very quiet.
"It is done now. You can't dwell on things like this because we have too many games and too little time to think about it.
"The League Cup has gone. The treble has gone. But we are still in three competitions and we have to progress as well as we can in those."