British light heavyweight Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight 10-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a fight where the techniques didn't complement each other, with excessive holding marring the fight.
Two judges had it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what defines champions? They win," the victor commented in the ringside as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Some ringside observers believed the challenger, thirty-one, merited the victory but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
The Champion Claims Victory in Dull Match
The fighter's advancement has stalled in the past few years through management issues and considerable time of lack of fights.
Once tipped as the future of UK boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers ready to take risks in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was participating in his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who utilized a tight guard to edge the better stanzas.
The audience's tolerance diminished because of frequent holding, with the challenger going down various instances during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates Manchester Derby Fight
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a terrific local rivalry battle to secure the European light-heavyweight championship.
The stadium became quiet for a deeply poignant tribute before the fight as a bell ceremony sounded in remembrance of Manchester's own Hatton, who died at age 46 in recently.
It was the first major televised boxing event in the location since Hatton's death. As the final chime ended, the quiet became a roar of "only one Hatton".
The fighting within the ring equaled the emotion. The eventual winner was floored by a powerful punch flush on the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was damaged early, heavy swelling developing under his face as the fighters traded punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur edged the verdict with scoring tallies of draw, 115-113 and 115-112, but each competitor were persistent and brave, all the qualities their inspiration the legend embodied.
