British light-heavyweight Buatsi earned a win yet underwhelming beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a competitive ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling individual punches in a match where the styles failed to mesh, with frequent clinching disrupting the fight.
The officials scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest display but what do champions do? They prevail," Buatsi commented in the arena as he was met by disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts thought Parker, 31, merited the victory but the Derbyshire fighter suffered the second loss of his 28-match professional journey.
The Champion Secures Victory in Slow Encounter
The fighter's advancement has plateaued in recent years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who worked behind a solid defense to edge the more effective rounds.
The spectators' acceptance wore thin during frequent holding, with Parker slipping to the canvas various instances during grappling.
The underdog closed well, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the supporting bout, Arthur overcame Rea by divided decision in a outstanding all-Manchester contest to secure the European light-heavyweight title.
The stadium became quiet for a profoundly moving homage prior to the bout as ten bells rang in memory of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the last bell faded, the quiet became a roar of "there's just one Ricky Hatton".
The fighting inside the ropes reflected the feeling. Arthur was sent to the canvas by a significant blow directly to the face in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured soon, significant bruising appearing under his orbital area as the pair traded punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the pace seldom decreased over twelve rounds.
Arthur edged the result with official scores of 114-114, 115-113 and 115-112, but the two athletes were persistent and full of heart, all the qualities their hero the legend stood for.
