British light-heavyweight Buatsi secured a victory yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight 10-round bout at the boxing arena.
But Parker had his moments, landing some telling clean blows in a contest where the approaches didn't complement each other, with excessive holding disrupting the action.
Two judges had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but what defines champions? They prevail," Buatsi stated in the arena as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Various analysts thought Parker, 31, deserved the decision but the contender endured the second defeat of his 28-bout professional journey.
The Champion Secures Triumph in Uneventful Encounter
The fighter's advancement has plateaued in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what seems like a last chance - under recent support.
He started carefully, with the two boxers prepared to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to barely take the more effective stanzas.
The spectators' acceptance wore thin because of regular grabbing, with Parker slipping to the canvas several times during grappling.
Parker finished the stronger, but multiple periods were challenging to assess. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Contest
In the co-main event, Arthur defeated Bradley Rea by divided decision in a excellent all-Manchester contest to win the European light-heavyweight championship.
The arena fell silent for a profoundly moving homage ahead of the contest as 10 bells rang in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised fight night in the city since the passing of Hatton. As the concluding sound ended, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The action during the bout matched the emotion. The eventual winner was sent to the canvas by a left hook flush on the face in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red shorts of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was injured soon, significant bruising appearing under his face as the pair exchanged punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the verdict with judges scorecards of 114-114, 115-113 and 115-112, but each competitor were relentless and courageous, all the qualities their idol the legend embodied.
