British light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a contest where the techniques failed to mesh, with excessive holding disrupting the action.
Two judges had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest display but what do champions do? They succeed," Buatsi said in the ringside as he was faced disapproval from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, deserved the decision but the British boxer experienced the second defeat of his 28-fight career.
The Champion Earns Victory in Slow Affair
Buatsi's progress has stalled in recent years through management issues and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers ready to be aggressive in a calculated early stages.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who operated with a protective stance to narrowly win the more effective rounds.
The audience's tolerance decreased during frequent holding, with Parker falling to the floor various instances during clinches.
Parker closed well, but multiple periods were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Motivates Manchester Derby Fight
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to capture the European light-heavyweight belt.
The stadium became quiet for a extremely emotional homage ahead of the contest as a bell ceremony sounded in memory of the city's native Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the concluding sound ended, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The competition within the ring matched the sentiment. Arthur was sent to the canvas by a left hook directly to the face in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was damaged early, significant bruising appearing under his face as the fighters landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the decision with judges scorecards of 114-114, narrow lead and clearer margin, but the two athletes were persistent and brave, all the qualities their hero Hatton stood for.
