UK light heavyweight Buatsi secured a victory yet underwhelming start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the styles failed to mesh, with constant grabbing marring the action.
The scoring judges scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They succeed," Buatsi said in the ring as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a twentieth professional win.
Various analysts thought Parker, 31, should have been awarded the victory but the contender endured the second defeat of his 28-fight career.
The Champion Claims Victory in Slow Affair
Buatsi's progress has plateaued in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to realize his potential - maybe in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors prepared to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who operated with a tight guard to edge the cleaner rounds.
The spectators' acceptance decreased amid regular grabbing, with the challenger slipping to the canvas several times during clinches.
Parker closed well, but several stanzas were hard to judge. Although, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Arthur defeated Rea by divided decision in a excellent all-Manchester fight to secure the continental title.
The venue became quiet for a profoundly moving tribute prior to the bout as a bell ceremony rang in remembrance of local hero Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the last bell faded, the quiet became a roar of "only one Ricky Hatton".
The competition within the ring reflected the sentiment. Arthur was knocked down by a significant blow directly to the face in the second round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was marked up soon, heavy swelling forming under his eye as the fighters exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the verdict with official scores of 114-114, narrow lead and wider gap, but both fighters were persistent and brave, exactly what their idol the legend stood for.
