UK light heavyweight Buatsi made a winning yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round fight at the boxing arena.
But Parker found success, connecting with powerful individual punches in a contest where the approaches didn't complement each other, with constant grabbing marring the contest.
Two judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," the victor stated in the ring as he was met by disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Various analysts believed the challenger, 31, deserved the victory but the British boxer suffered the second setback of his 28-bout career.
Buatsi Claims Triumph in Slow Affair
His career development has plateaued in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers prepared to be aggressive in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to barely take the better rounds.
The audience's tolerance wore thin during frequent holding, with Parker slipping to the canvas multiple occasions during grappling.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Inspires Manchester Derby Fight
In the supporting bout, Arthur overcame Rea by split-decision in a excellent Manchester vs Manchester battle to win the European light-heavyweight championship.
The arena grew hushed for a deeply poignant memorial ahead of the contest as 10 bells tolled in remembrance of the city's native Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the concluding sound faded, the silence gave way to a shout of "there's only one Hatton".
The action inside the ropes equaled the emotion. Arthur was knocked down by a left hook directly to the chin in the early round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was injured soon, heavy swelling forming under his eye as the fighters landed punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over the full distance.
Arthur barely secured the decision with official scores of 114-114, narrow lead and clearer margin, but each competitor were determined and full of heart, everything their idol the legend embodied.
