British light-heavyweight Buatsi earned a win yet disappointing beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close ten-round contest at the boxing arena.
But the challenger found success, landing some telling single shots in a match where the styles failed to mesh, with excessive holding disrupting the action.
The officials scored it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best performance but what do champions do? They succeed," Buatsi stated in the ringside as he was faced boos from the crowd.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts thought Parker, thirty-one, merited the win but the contender endured the second loss of his 28-bout career.
The Champion Secures Triumph in Slow Affair
Buatsi's progress has plateaued in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what seems like a final roll of the dice - under recent support.
He began cautiously, with neither fighter ready to overcommit in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who operated with a protective stance to barely take the cleaner stanzas.
The audience's tolerance diminished amid frequent holding, with Parker going down multiple occasions during clinches.
Parker ended stronger, but many rounds were hard to judge. However, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires Local Rivalry Fight
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific all-Manchester fight to secure the European light-heavyweight title.
The venue fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound ended, the silence became a chant of "only one Ricky Hatton".
The action inside the ropes matched the sentiment. The eventual winner was knocked down by a left hook directly to the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the Manchester United-red attire of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was marked up soon, heavy swelling forming under his orbital area as the duo landed punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur barely secured the verdict with scoring tallies of draw, 115-113 and 115-112, but both fighters were determined and full of heart, all the qualities their hero the legend represented.
