UK 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 activity level appeared to earn him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the techniques didn't complement each other, with constant grabbing disrupting the contest.
Two judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my greatest display but what do champions do? They win," Buatsi stated in the arena as he was greeted with boos from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers believed the challenger, thirty-one, merited the victory but the British boxer experienced the second setback of his 28-fight career.
The Champion Secures Victory in Uneventful Affair
Buatsi's progress has plateaued in recent years through promotional disputes and considerable time of lack of fights.
Once tipped as the future of the British fight scene, he now is positioned trying to realize his potential - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors ready to overcommit in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who utilized a protective stance to narrowly win the better rounds.
The crowd's patience diminished because of frequent holding, with the challenger falling to the floor several times during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. Nevertheless, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Hatton Homage Inspires Manchester Derby Contest
In the supporting bout, Arthur overcame Rea by close verdict in a outstanding Manchester vs Manchester battle to win the European light heavyweight title.
The venue fell silent for a extremely emotional homage prior to the bout as a bell ceremony sounded in memory of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the concluding sound faded, the silence transitioned to a shout of "there's just one Hatton".
The fighting inside the ropes reflected the emotion. Arthur was knocked down by a powerful punch squarely on the jaw in the second round, standing uncertainly to his footing on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was damaged early, heavy swelling appearing under his eye as the duo traded hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor edged the result with scoring tallies of 114-114, 115-113 and clearer margin, but both fighters were determined and brave, everything their idol Hatton represented.
