Motherwell returned to second place in the Clydesdale Bank Premier League with a positive display against the runaway leaders that was capped by Michael Higdon's 73rd-minute winner.
Lennon excluded James Forrest from criticism and the winger felt it was just an off night.
Celtic rarely troubled Darren Randolph before Georgios Samaras headed an equaliser in the 63rd minute to cancel out Chris Humphrey's opener, but the Motherwell goalkeeper was a busier man in the final 15 minutes.
Forrest said: "We know that they are second or third in the league and we were all up for the game. It just wasn't our night.
"I thought we created enough chances to win the game. They had two or three shots and scored two, so we were a bit unlucky."