British light heavyweight Buatsi secured a victory yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a close ten-round fight at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a match where the styles never quite clicked, with frequent clinching spoiling the action.
The officials scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best performance but what do champions do? They succeed," the victor said in the arena as he was met by disapproval from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers believed the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter suffered the second loss of his 28-match professional journey.
The Champion Claims Triumph in Slow Affair
Buatsi's progress has plateaued in recent years through management issues and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers willing to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who utilized a tight guard to narrowly win the more effective stanzas.
The spectators' acceptance diminished amid frequent holding, with the challenger falling to the floor several times during close exchanges.
The underdog finished the stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a outstanding local rivalry contest to capture the European light-heavyweight championship.
The arena became quiet for a extremely emotional homage ahead of the contest as a bell ceremony tolled in honor of the city's native Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound faded, the silence transitioned to a shout of "only one Ricky Hatton".
The action during the bout matched the feeling. The eventual winner was knocked down by a powerful punch squarely on the face in the second round, standing uncertainly to his feet on the eight count.
But the traditional red attire of Arthur soon came forward again. From the other corner, the opponent - in rival City blue - was injured quickly, significant bruising developing under his face as the pair landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur edged the result with judges scorecards of 114-114, narrow lead and clearer margin, but each competitor were determined and courageous, exactly what their inspiration Hatton embodied.
