UK light heavyweight Buatsi earned a win yet disappointing start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a close ten-round fight at the boxing arena.
But the challenger had his moments, delivering effective single shots in a fight where the techniques failed to mesh, with frequent clinching disrupting the fight.
The officials had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest performance but how do champions respond? They prevail," the victor commented in the ringside as he was faced negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the victory but the Derbyshire fighter experienced the second loss of his 28-fight professional journey.
Buatsi Secures Triumph in Uneventful Match
Buatsi's progress has stalled in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with both competitors ready to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second fight at light heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who utilized a tight guard to edge the more effective rounds.
The spectators' acceptance diminished amid frequent holding, with the challenger going down several times during clinches.
Parker closed well, but multiple periods were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor moves on, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Fuels Local Rivalry Contest
In the co-main event, Arthur defeated Bradley Rea by divided decision in a excellent local rivalry fight to secure the European light heavyweight championship.
The venue fell silent for a deeply poignant homage ahead of the contest as ten bells sounded in remembrance of local hero Hatton, who died aged 46 in September.
It was the first major televised match card in the city since Hatton's passing. As the final chime faded, the stillness gave way to a shout of "there's only one Hatton".
The action inside the ropes equaled the feeling. Arthur was knocked down by a left hook flush on the face in the initial stage, standing uncertainly to his feet on the referee's count.
But the traditional red attire of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was injured soon, significant bruising developing under his face as the duo landed hard punches.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the pace seldom decreased over the full distance.
Arthur narrowly won the verdict with scoring tallies of draw, close margin and 115-112, but the two athletes were relentless and courageous, exactly what their hero the legend embodied.
