British light-heavyweight Buatsi secured a victory yet underwhelming start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight ten-round fight at the boxing arena.
But the challenger found success, delivering effective individual punches in a fight where the techniques never quite clicked, with excessive holding disrupting the fight.
The scoring judges scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my greatest showing but what defines champions? They succeed," Buatsi stated in the ring as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in February - to earn a twentieth professional win.
Various analysts felt Parker, thirty-one, should have been awarded the decision but the British boxer endured the second defeat of his 28-match career.
Buatsi Secures Triumph in Uneventful Affair
Buatsi's progress has stalled in recent years through management issues and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to achieve expectations - maybe in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who worked behind a tight guard to edge the cleaner rounds.
The audience's tolerance decreased because of constant clinching, with the challenger slipping to the canvas several times during close exchanges.
The underdog closed well, but several stanzas were hard to judge. However, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor advances, but with the likes of power puncher Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the supporting bout, Arthur beat Rea by close verdict in a terrific local rivalry fight to capture the European light heavyweight belt.
The venue became quiet for a extremely emotional tribute prior to the bout as 10 bells rang in remembrance of the city's native Ricky Hatton, who died 46 years old in September.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound faded, the silence became a chant of "there's just one Hatton".
The action inside the ropes reflected the emotion. Arthur was knocked down by a significant blow flush on the face in the early round, rising shakily to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was marked up quickly, heavy swelling developing under his orbital area as the pair exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the pace rarely dropped over the full distance.
The victor edged the result with judges scorecards of 114-114, close margin and wider gap, but both fighters were persistent and full of heart, exactly what their inspiration Hatton embodied.
