UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a competitive 10-round bout at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a contest where the styles didn't complement each other, with constant grabbing marring the contest.
The scoring judges scored it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my best showing but how do champions respond? They win," the victor said in the ring as he was faced negative reactions from the crowd.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, should have been awarded the victory but the British boxer suffered the second defeat of his 28-match professional journey.
The Champion Claims Victory in Uneventful Match
Buatsi's progress has slowed in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at light-heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a tight guard to narrowly win the better rounds.
The crowd's patience wore thin amid constant clinching, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but many rounds were hard to judge. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor advances, but with the presence of power puncher Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a excellent all-Manchester battle to win the continental title.
The arena became quiet for a deeply poignant tribute before the fight as a bell ceremony rang in remembrance of the city's native Hatton, who passed away aged 46 in September.
It was the premier televised fight night in the city since Hatton's death. As the concluding sound ended, the silence transitioned to a shout of "there's only one Ricky Hatton".
The competition within the ring equaled the sentiment. The eventual winner was knocked down by a significant blow directly to the chin in the early round, rising shakily to his feet on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged quickly, heavy swelling developing under his face as the pair landed punishing blows.
The challenger, 27, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over the full distance.
The victor narrowly won the result with official scores of draw, close margin and clearer margin, but the two athletes were determined and full of heart, all the qualities their hero the legend stood for.
