British light heavyweight Buatsi secured a victory yet disappointing start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a close ten-round contest at the boxing arena.
But the challenger found success, delivering effective single shots in a contest where the styles didn't complement each other, with excessive holding spoiling the action.
Two judges scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best performance but what defines champions? They prevail," the victor said in the arena as he was greeted with boos from the crowd.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Various analysts thought Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second setback of his 28-match career.
The Champion Claims Triumph in Slow Match
Buatsi's progress has slowed in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to take risks in a tentative initial rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to narrowly win the better rounds.
The crowd's patience diminished because of constant clinching, with Parker falling to the floor various instances during close exchanges.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to secure the European light heavyweight belt.
The venue grew hushed for a deeply poignant memorial before the fight as a bell ceremony sounded in honor of Manchester's own Hatton, who died aged 46 in recently.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound faded, the silence transitioned to a shout of "there's just one Hatton".
The competition inside the ropes reflected the emotion. Arthur was knocked down by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was injured quickly, heavy swelling forming under his eye as the fighters exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the result with judges scorecards of even score, close margin and 115-112, but the two athletes were relentless and courageous, everything their hero the legend represented.
