UK light heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a tight ten-round contest at the Manchester venue.
But Parker found success, landing some telling individual punches in a contest where the approaches failed to mesh, with frequent clinching disrupting the action.
The scoring judges had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," the victor stated in the ring 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 last winter - to secure a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the win but the contender endured the second loss of his 28-bout career.
Buatsi Secures Win in Uneventful Match
His career development has slowed in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a last chance - under fresh management.
He opened tentatively, with neither fighter willing to take risks in a calculated early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was participating in his second appearance at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who operated with a tight guard to barely take the better rounds.
The spectators' acceptance diminished amid regular grabbing, with Parker slipping to the canvas multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were hard to judge. However, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a excellent local rivalry battle to secure the continental championship.
The stadium fell silent for a deeply poignant tribute prior to the bout as ten bells rang in memory of the city's native Hatton, who died aged 46 in recently.
It was the premier televised fight night in the city since Hatton's death. As the final chime faded, the quiet transitioned to a chant of "only one Hatton".
The fighting inside the ropes equaled the emotion. The eventual winner was floored by a significant blow squarely on the jaw in the early round, rising shakily to his position on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was damaged soon, heavy swelling developing under his orbital area as the pair traded punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over the full distance.
Arthur narrowly won the result with official scores of 114-114, 115-113 and clearer margin, but each competitor were relentless and courageous, exactly what their inspiration the legend stood for.
