"I don't know. I don't know the answer to that," he told CNN's Dana Bash when asked why Trump let him go.
He also heaped praise on his former boss and insisted he will do "everything" he can to make sure Trump is elected president, whether from "inside" or "outside" the campaign.
"I think Donald Trump is going to be the next president of the United States. If there's anything I can do to help make that happen ... I'll be happy to do that," Lewandowski said.
He said he told Trump on Monday "it's been an honor and a privilege" to work for him.
When asked what his biggest regret of the campaign was, Lewandowski said" "None, professionally."
But Lewandowski conceded that his departure came at a time when the campaign needs to "evolve" and "transition" past the lean operation that helped Trump clinch the Republican nomination and into an organization that can compete with what Lewandowski called Hillary Clinton's "very well-funded campaign."
"I'm proud of the campaign -- I really am -- but I also understand the reality of building an infrastructure coupled with the (Republican National Committee's) 500 people on the ground," Lewandowski said.
In the nearly 30-minute interview with Bash, the sharp-elbowed operative heaped praise on those -- including Trump's children and campaign chairman Paul Manafort -- who urged Trump to fire him, according to multiple sources.
He described Trump's daughter, Ivanka, as "somebody who understands her father and is a fantastic surrogate for the campaign," and also praised her husband, Jared Kushner, who has become an increasingly influential adviser and who reportedly butted heads with Lewandowski.
"I've had a great relationship with Jared," Lewandowski said, adding that he had "no interest" in trying to plant negative stories against Kushner, which several sources told CNN Lewandowski had been doing.
Of Manafort, Lewandowski said: "Paul and I have gotten along amazingly well. We worked side-by-side. There was no animosity between Paul and I."
Sources inside and around Trump's campaign painted a starkly different picture, describing the power struggle between the two men as hindering the campaign's growth and causing disorder within the campaign.
And while Lewandowski denied allegations that he has been a "yes man" of sorts with Trump, he said the candidate "has had his finger on the pulse of the American public" throughout his campaign and that he only pushed back on "issues that are important."
"I don't just give a 'yes' answer," he said. "And I will fight for my opinions, forcefully."
Lewandowski didn't reject out of hand allegations from his former colleagues that he is a "hothead," but used a different word: "intense."
"I think I'm a very intense person and, um, my expectation is perfection because I think that's what Mr. Trump deserves," Lewandowski said. "When I see someone who I don't think is working as hard as (Trump)...yeah that bothers me."
Lewandowski said he will still chair the New Hampshire delegation to the Republican National Committee's summer convention where Trump is set to officially become the GOP nominee.