British light-heavyweight Buatsi secured a victory yet unimpressive start to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But Parker found success, landing some telling clean blows in a fight where the styles failed to mesh, with excessive holding spoiling the action.
The officials scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They succeed," the victor commented in the ringside as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th career victory.
Some ringside observers thought Parker, thirty-one, merited the victory but the Derbyshire fighter endured the second defeat of his 28-bout career.
The Champion Secures Win in Uneventful Match
Buatsi's progress has plateaued in recent years through management issues and considerable time of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers willing to be aggressive in a calculated initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was making just his second fight at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who operated with a protective stance to edge the better periods.
The spectators' acceptance decreased amid regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
The underdog ended stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Homage Motivates Manchester Derby Fight
In the supporting bout, Arthur beat Rea by close verdict in a excellent local rivalry fight to win the European light heavyweight belt.
The arena became quiet for a profoundly moving homage ahead of the contest as 10 bells tolled in remembrance of the city's native Ricky Hatton, who died 46 years old in September.
It was the first major televised boxing event in the city since Hatton's passing. As the concluding sound ended, the stillness transitioned to a chant of "only one Ricky Hatton".
The action within the ring equaled the sentiment. The eventual winner was knocked down by a left hook squarely on the face in the initial stage, getting up unsteadily to his position on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged quickly, heavy swelling developing under his eye as the fighters exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over twelve rounds.
Arthur barely secured the decision with official scores of even score, close margin and clearer margin, but each competitor were relentless and courageous, exactly what their hero the legend embodied.
