UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close 10-round bout at the boxing arena.
But the challenger found success, landing some telling individual punches in a fight where the approaches never quite clicked, with excessive holding spoiling the action.
The scoring judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest showing but what defines champions? They succeed," the victor commented in the ring as he was greeted with disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts thought Parker, thirty-one, merited the decision but the contender suffered the second setback of his 28-bout professional journey.
Buatsi Secures Triumph in Slow Match
His career development has stalled in the past few years through contract problems and considerable time of lack of fights.
Previously predicted as the next star of the British fight scene, he now is positioned trying to achieve expectations - possibly in what feels like a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who operated with a tight guard to edge the cleaner stanzas.
The spectators' acceptance wore thin during frequent holding, with Parker going down various instances during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. Although, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Memorial Fuels All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a terrific all-Manchester fight to win the continental title.
The stadium fell silent for a deeply poignant memorial before the fight as a bell ceremony tolled in memory of the city's native Hatton, who died aged 46 in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the final chime ended, the silence gave way to a shout of "there's only one Hatton".
The fighting inside the ropes equaled the feeling. Arthur was sent to the canvas by a powerful punch squarely on the chin in the second round, standing uncertainly to his position on the referee's count.
But the traditional red attire of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was marked up early, noticeable damage developing under his eye as the pair landed powerful shots.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur barely secured the decision with official scores of 114-114, close margin and wider gap, but the two athletes were relentless and brave, exactly what their idol Hatton represented.
