Keegan wrote in his ESPN Soccernet column: 'Newcastle need some stability, but you will never get stability when you have Mike Ashley and Derek Llambias running the club.'
Doomed relationship: Keegan (right) with Ashley in his Newcastle days
His outburst came as new boss Alan Pardew prepared to face a frosty reception on his first game in charge.
Newcastle fans were already planning to protest at the Hughton sacking ahead of today's clash with Liverpool at St James' Park.
Keegan's comments could provoke an even more furious reaction aimed at those in charge of the club.
In his column Keegan, who was hired by owner Mike Ashley and his managing director Derek Llambias and quit in September 2008 after just eight months, continued: 'Ashley has made no secret that he wants to sell Newcastle, and the best day will come when someone buys it from him and runs Newcastle like that club should be run.
'The club can never go anywhere under Mike Ashley and Derek Llambias.
'I don't understand the decision to get rid of Chris. I lived with the same people for eight months - I didn't understand them then and I don't understand them now, and I don't think many of the fans do.
'When a man buys a club like Mike Ashley did, and knows as little about football as he does, he is likely to make these sort of mistakes and these judgments and still think he is doing OK.
Happy days: Hughton lifts the trophy after winning the Championship with Newcastle last season
'When you get the fans saying it is the wrong decision, when you get the players saying it is wrong and when you even get the press saying it is wrong, it tells you, unsurprisingly, that the decision is the wrong one.
'The news won't have come as a total surprise to Chris, having worked with the people he had been working with for the best part of two-and-a-bit years. He knew what he was dealing with.
'It's almost impossible to work in that environment, and that's why I take my hat off to Chris for getting on with it for as long as he did.
'He did a great job under difficult circumstances, with very little investment, and his reward for that is for them to say, "Thanks very much Chris, you did a great job but we don't want you".
'The decision is not a reflection on his work at Newcastle, it is a sad reflection on the people running the club.
'Not many people understand the rationale behind the decision.'
Off on the wrong foot: Alan Pardew at Newcastle
Pardew held his first training session with his new players behind closed doors yesterday, avoiding photographers, supporters and the ice and snow on the outdoor pitches.
All eyes will be on the reaction from fans, who have inundated websites and forums with criticisms throughout the week since Hughton was relieved of his post.
Former West Ham and Charlton boss Pardew may find himself caught in the crossfire this evening and Keegan is sympathetic to his plight.
He added: 'Alan's a good lad. I know him well, he is dedicated and he works very hard. The sad thing is that he is getting off on the wrong foot through no fault of his own.
'The supporters won't be resistant to Alan because of the circumstances of his appointment, though.
'Any criticism will be levelled at Llambias and Mike Ashley, because it is their decision. The fans would be wrong to pick on Alan.
'Obviously if results don't go well then they may start to voice discontent, but Alan is just the meat in the middle of the sandwich here.'
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Explore more:People: Mike Ashley, Chris Hughton, Kevin Keegan, Derek Llambias Places: Newcastle, Liverpool