British light heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a contest where the techniques failed to mesh, with excessive holding marring the action.
The officials scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest showing but how do champions respond? They win," Buatsi said in the ringside as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Some ringside observers believed Parker, thirty-one, deserved the victory but the contender suffered the second loss of his 28-fight career.
Buatsi Secures Win in Dull Encounter
The fighter's advancement has plateaued in recent times through management issues and long spells of inactivity.
Previously predicted as the future of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a last chance - under fresh management.
He started carefully, with neither fighter prepared to take risks in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who utilized a tight guard to edge the cleaner rounds.
The spectators' acceptance diminished amid frequent holding, with the challenger going down various instances during clinches.
Parker finished the stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Arthur beat Rea by split-decision in a excellent local rivalry battle to win the European light heavyweight belt.
The arena fell silent for a deeply poignant tribute ahead of the contest as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the last bell ended, the quiet gave way to a roar of "only one Hatton".
The action within the ring reflected the sentiment. The eventual winner was floored by a powerful punch directly to the chin in the initial stage, rising shakily to his position on the eight count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up early, heavy swelling appearing under his face as the pair landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over the full distance.
The victor barely secured the decision with official scores of 114-114, narrow lead and 115-112, but each competitor were persistent and brave, everything their hero the legend embodied.
