UK light-heavyweight Buatsi earned a win yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive 10-round bout at the boxing arena.
But the challenger had his moments, landing some telling clean blows in a fight where the approaches didn't complement each other, with excessive holding spoiling the action.
The officials had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest performance but what defines champions? They win," Buatsi said in the arena as he was greeted with negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, deserved the decision but the Derbyshire fighter endured the second defeat of his 28-match professional journey.
The Champion Secures Triumph in Slow Affair
His career development has stalled in recent years through contract problems and considerable time of lack of fights.
Previously predicted as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers prepared to overcommit in a tentative opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who operated with a tight guard to edge the cleaner stanzas.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas several times during clinches.
Parker ended stronger, but multiple periods were difficult to score. However, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Contest
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a outstanding Manchester vs Manchester contest to capture the continental belt.
The venue grew hushed for a profoundly moving tribute prior to the bout as ten bells rang in memory of the city's native Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised fight night in the location since the passing of Hatton. As the concluding sound disappeared, the silence gave way to a roar of "there's only one Hatton".
The action within the ring reflected the emotion. The eventual winner was knocked down by a powerful punch flush on the jaw in the early round, getting up unsteadily to his position on the count of eight.
But the traditional red shorts of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was marked up quickly, significant bruising forming under his face as the fighters landed punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of 114-114, 115-113 and 115-112, but each competitor were determined and full of heart, exactly what their inspiration the legend embodied.
