The Hoops crashed 2-0 to Heart of Midlothian at Tynecastle with Commons' sanction coming in the 65th minute when the score was 1-0.
"We lost our discipline. Well, one player lost his discipline and let us down badly," Lennon said.
"I didn't really get a good look at it but, from what I gather, it's a lunge and it's high.
"He has come in and held his hand up - but it's too late.
"We're all disappointed. He's let himself down and he's let us down."