British light heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful individual punches in a match where the approaches failed to mesh, with frequent clinching disrupting the action.
Two judges had it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my best display but what do champions do? They succeed," Buatsi stated in the arena as he was faced boos from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought Parker, 31, should have been awarded the victory but the contender endured the second loss of his 28-bout professional journey.
Buatsi Claims Victory in Slow Affair
Buatsi's progress has slowed in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what feels like a last chance - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at light-heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for the favorite, who operated with a solid defense to barely take the better periods.
The spectators' acceptance decreased amid frequent holding, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but many rounds were difficult to score. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor advances, but with the caliber of hard hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester contest to secure the European light-heavyweight belt.
The arena became quiet for a deeply poignant homage ahead of the contest as a bell ceremony sounded in memory of local hero Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the urban center since the passing of Hatton. As the final chime faded, the stillness transitioned to a shout of "only one Ricky Hatton".
The fighting within the ring equaled the feeling. Arthur was floored by a left hook flush on the chin in the initial stage, rising shakily to his footing on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured soon, significant bruising forming under his eye as the duo exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur edged the decision with judges scorecards of 114-114, narrow lead and clearer margin, but the two athletes were persistent and full of heart, everything their hero the legend represented.
