Ngannou (11-3) lost to Lewis by unanimous decision in one of the most inactive contests in UFC history. Ngannou landed 11 total strikes in the fight, and both heavyweights were warned for timidity by referee Herb Dean.
Ngannou said Monday he was still feeling the effects of a five-round decision loss to Stipe Miocic in January.
"I am not proud of my last performance," the French-Cameroonian wrote on social media. "I have carried my fear from the last fight to this one. I completely understand the frustration and anger that it has caused to my fans, coaches, teammates, family and friends and I am truly sorry for that.
"I won't let everyone down again. All I can do now is prove myself and make you proud again."
Ngannou, who trains out of Las Vegas, won his first six fights in the UFC before he was outclassed by Miocic in a failed title bid.
Lewis (20-5) was also apologetic about his performance. He landed 20 total strikes and admitted his back was injured during the fight.
"I believe it hurt me more than it helped me, even though I got the win," Lewis said at the postfight news conference. "I believe it probably set me two fights back. My back was killing me. All I wanted to do was sit down and stay on that bench."
UFC president Dana White referred to the bout, which co-headlined UFC 226, as "the worst thing I've ever seen in my life."