While the exact reason for Keith Olbermann’s departure from MSNBC was the source of much speculation over the weekend, his exit poses a challenge for the newsie and even for incoming corporate parent Comcast.

Olbermann hosted the highest-rated show on MSNBC, and when he abruptly announced at the end of his show Friday that it would be his last, the network quickly issued a press release juggling its schedule and announcing that Lawrence O’Donnell, who launched “The Last Word” in September, would be taking the 8 p.m. timeslot of “Countdown WithKeith Olbermann.”

There were reports and indications that his departure had been in the works for days or even weeks, but it still came as a surprise to followers, some of whom declared that they would stop watching the network in protest.

It was Olbermann, more than any other figure, who drove MSNBC’s rebranding into a leftward alternative to Fox News, making him a nightly must-watch for progressives, particularly when he would deliver his “special comment” segments, unsettling to some traditionalists on the NBC News side.

O’Donnell has a point of view, but he’s less biting in his rhetoric and has delivered a more traditional cross section of partisan guests than “Countdown.” Rachel Maddow also has been a breakout primetime personality for MSNBC since her show was launched in 2008 (she’s remaining in her 9 p.m. ET slot), and the network is moving Ed Schultz, a favorite among progressives, to 10 from 6 p.m., which could elevate his profile.

On CNN’s “Reliable Sources” on Sunday, former MSNBC anchor and reporter David Shuster said, “There are so many progressives out there who felt like he was the poetic, literary, intelligent emotion and passionate voice for them, and they are looking up now and saying, ‘OK, where do we turn now? Is it going to be an Internet venture? Is it going to be on the radio? Is it going to be maybe with Rachel or Ed (Schultz) or somebody else at MSNBC?’ But they are missing this voice.”

MSNBC insiders said that discussions between the network and Olbermann had been going on for some time, even though he had at least two years on his existing contract.

The friction flared up publicly when Olbermann was briefly suspended in November after it was disclosed that he had contributed to congressional candidates, a violation of the standards of the news division. He returned two days later, but at the time, some observers predicted that the flap would lead to his eventual departure.

The network did not give a reason for Olbermann’s exit other than to say that “MSNBC and Keith Olbermann have ended their contract.”

It also was unclear whether it was the network or Olbermann that sought his departure. In his final signoff he invoked the 1976 movie “Network,” about a news anchor from the left whose commentary becomes too hot for the network to handle. He spoke of being told “that this is going to be the last edition of your show.”

He also thanked a long list of colleagues past and present, as well as viewers, and called the late Tim Russert “my greatest protector and the most indefatigable cheerleader.” Noticeably absent was any mention of MSNBC president Phil Griffin.

Shuster suggested that the change in reporting structure that will occur when Comcast takes control of NBC could have played a part in Olbermann’s exit. Griffin, who had been reporting to departing NBC Universal chieftain Jeff Zucker, now will report to NBC News prexy Steve Capus.

“And so NBC News is going to have much more of an influence over what happens on MSNBC,” Shuster said on “Reliable Sources.” “And I think Keith anticipated, perhaps justifiably so, that his wings might be clipped, that (in) some of the special commentaries that he would be making, that there would be much more sort of deference that would have to be paid to NBC News’ standards and judgments.”

When Olbermann was suspended, more than 250,000 fans signed a petition for his reinstatement.

Citing conversations he has had with MSNBC staffers, Shuster said Olbermann was “mesmerized beyond belief” by the outpouring of support, and it could have motivated him to want to take that fanbase to “a forum were I’ll have the kind of independence that I’ve always wanted.”

CNN would be an option, but according to the New York Times, Olbermann is restricted in making an immediate move to a competitor, as Conan O’Brien was after he reached a settlement for his departure last year from “The Tonight Show.”

Christopher Sterling, a professor of media and public affairs at George Washington U., said that he suspects Olbermann’s suspension from MSNBC “was the final trigger.”

“All this suggests (it’s) more about Olbermann than a systemic issue with the cable network or NBC News more broadly,” he said.

MSNBC insiders were quick to say Comcast execs had “nothing to do” with Olbermann’s departure and were informed of the decision Tuesday, after the FCC and Justice Dept. gave regulatory approval to the merger of Comcast and NBC U, with the deal expected to close by month’s end.

Comcast swiftly issued a statement also denying its involvement.

“Comcast pledged from the day the deal was announced that we would not interfere with NBC Universal’s news operations,” a company spokeswoman said. “We have not and will not.”

Dan Kennedy, an assistant journalism professor at Northeastern U. in Boston, has raised questions about the merger’s impact on the news business.

But he said, “If this happened a year from now, it would be one thing. But I can’t imagine Comcast executives would be so politically tone-deaf that they would whack Olbermann before they had ever completed their takeover.”