Boro confirmed this week that Karanka will remain in charge, despite walking out over the weekend and missing Sunday's 2-0 defeat at second-bottom Charlton Athletic.
It was reported by the Daily Mail that Karanka was finding it difficult to deal with Boro favourite Stewart Downing, whom it is claimed the Spaniard did not want to sign last summer, and his future at the Riverside looked to be in doubt.
Since resuming his duties at Boro, who can go second in the Championship with a win against Hull city on Friday, Karanka has called for togetherness from top to bottom, though Hamann told talkSPORT: "It's a bit like Lance Armstrong taking over at WADA."
That was merely a snippet of what was a scathing attack by the German, who also compared Karanka to his former Real Madrid colleague Jose Mourinho.
"I’ve never seen a more selfish act in a managerial [sense] than Aitor Karanka's in the last two or three weeks," Hamann began on Wednesday's Colin Murray show.
"He walked out; didn’t turn up at the training ground for a couple of days; didn’t go with the team to Charlton - a massive, massive game because it [a win] would have cemented their position in the top two.
"If that [the Downing signing] is an issue, it should have been to light in September or October - not in March.
"They’re in a fantastic position to go up automatically. Just imagine the buzz around Middlesbrough; the difference it makes to the city, creating jobs; what it makes to the football club; for the people; the feel-good factor. It’s massive."
And given the predicament of Newcastle and Sunderland, Hamann feels there has never been a better opportunity for Boro to gain promotion.
"You’ve got two clubs down the road who are fighting relegation, who are in completely the opposite boat. They might be going down, the pair of them, and Middlesbrough could go up; back in the Premier League, where I think they belong.
"He’s got nothing better to do than think about himself; put himself first. He’s thrown his toys out of the pram saying ‘this is not how I want it’. Without him they lost three points away to Charlton.
"We shouldn’t forget he is a very close ally of Jose Mourinho - maybe this is where he gets it from? I’ve not seen anything in a leading position, as a manager in football, to put the success of the team in doubt by a more selfish act than this man."