Ousted Defense Secretary Chuck Hagel had become "very, very frustrated" in the post before President Obama sought his resignation, according to a key lawmaker, who blasted the White House for a whispering campaign that accompanied the Pentagon boss' abrupt ouster Monday morning.

“I thank Chuck Hagel for his service, and I know that he was very, very frustrated,” Sen. John McCain, the Arizona Republican who is expected to take control of the Senate Armed Services Committee in January, said in a radio interview with KFYI.

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McCain spoke following a Rose Garden press conference at which Hagel stood stiffly while Obama announced he would be stepping down. Although McCain, who served in the Senate with Hagel from 1996-2008, opposed his fellow Vietnam veteran's appointment to defense secretary, he said administration sources were wrong to assail Hagel on his way out the door.

“Already White House people are leaking, ‘Well, he wasn’t up to the job,’" McCain said. "Well, believe me, he was up to the job. It was the job he was given, where he really was never really brought into that real tight circle inside the White House that makes all the decisions which has put us into the incredible debacle that we’re in today throughout the world.”

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McCain noted that Hagel characterized the Islamic State as the greatest threat in the Middle East, while Obama was calling them the jayvee (junior varsity) team less than a year ago. He also criticized Obama for failing in areas like the Middle East, Ukraine, and responding to a newly aggressive China.

“We’ve had our disagreements but Chuck Hagel is an honorable man,” McCain added.

At the news conference, Obama said Hagel concluded it’s an “appropriate time” for him to complete his service. Hagel has had a rocky tenure of nearly two years in which he's struggled to break through the White House's insular foreign policy team. He stepped down under pressure amid multiple foreign policy crises, including the rise of the Islamic State group.

But in remarks at the White House, Obama praised Hagel as "an exemplary defense secretary" and steady hand for strategy and budget. Obama said he'll always be grateful that Hagel has always "given it to me straight."

The Vietnam veteran and former Republican senator took office less than two years ago, and was charged with overseeing the winding down of decade-long wars in Afghanistan and Iraq. Hagel, 68, also steered the military during sweeping changes involving gays and women in the military. But in recent months, the Pentagon has taken on new challenges, including fighting Islamic State in Syria and Iraq and deploying military personnel to Africa to fight Ebola.

Sources told FoxNews.com that Obama's dissatisfaction with Hagel, as well as a desire to shake up the cabinet following the devastating midterm elections, played a role in the president seeking Hagel's ouster.

“Make no mistake, Secretary Hagel was fired,” a senior U.S. official with close knowledge of the situation told Fox News.

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This same official discounted Pentagon claims it was a mutual decision claiming President Obama has lost confidence in Hagel and that the White House had been planning to announce his exit for weeks.

“The president felt he had to fire someone. He fired the only Republican in his cabinet. Who is that going to piss off that he cares about?"

In a swipe at the resume of Hagel, who served as U.S. Army sergeant in Vietnam and received two Purple Hearts, the official added, “This is why you don’t send a sergeant to do a secretary’s job.”

Hagel took office Feb. 27, 2013, five years after retiring from the Senate. Prior to his political career, Hagel co-founded Vanguard Cellular, worked for an investment banking firm and ran American Information Systems, a company that makes computerized voting machines. He also taught at Georgetown University after stepping down from the Senate.

A senior defense official said that Hagel submitted his resignation letter to Obama Monday morning and that the president accepted it. Hagel agreed to remain in office until his successor is confirmed by the Senate, the official said.

The president is not expected to nominate a new Pentagon chief Monday, according to one official.

The officials insisted on anonymity because they were not authorized to discuss the matter by name ahead of Obama's official announcement.

Hagel, the only Republican on Obama’s cabinet, served as senator from Nebraska for two terms, beginning in 1996, and became a critic of U.S. involvement in Iraq. Obama nominated him to succeed Leon Panetta as Defense Secretary in his second term.

Recent questions about Hagel's future at the Pentagon were prompted in part by his decision to postpone a long-planned trip this month to Vietnam. At the time, officials said he needed to remain in Washington for congressional consultations, but that did not stop speculation that the White House might be looking for a replacement for the final two years of Obama's term.

Just last week, Hagel was asked about the speculation during an interview on the Charlie Rose show. He was asked whether he's concerned by the speculation.

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"No. First of all, I serve at the pleasure of the president," Hagel said. "I'm immensely grateful for the opportunity I've had the last two years to work every day for the country and for the men and women who serve this country. I don't get up in the morning and worry about my job. It`s not unusual by the way, to change teams at different times."

Fox News' Justin Fishel and The Associated Press contributed to this report.