British light heavyweight Buatsi earned a win yet disappointing start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a close 10-round fight at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful single shots in a contest where the approaches failed to mesh, with excessive holding marring the action.
The officials had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They win," Buatsi stated in the arena as he was faced disapproval from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th career victory.
Various analysts felt Parker, 31, should have been awarded the win but the British boxer suffered the second defeat of his 28-match career.
The Champion Secures Win in Dull Encounter
The fighter's advancement has plateaued in recent years through management issues and extended periods of lack of fights.
Once tipped as the future of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a last chance - under fresh management.
He began cautiously, with the two boxers prepared to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a tight guard to narrowly win the cleaner stanzas.
The crowd's patience wore thin because of frequent holding, with the challenger going down multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Motivates Local Rivalry Battle
In the supporting bout, Arthur overcame Bradley Rea by split-decision in a excellent Manchester vs Manchester battle to win the continental belt.
The venue grew hushed for a extremely emotional homage ahead of the contest as ten bells sounded in remembrance of the city's native Hatton, who succumbed 46 years old in last month.
It was the first major televised match card in the city since the passing of Hatton. As the final chime disappeared, the silence transitioned to a roar of "there's just one Ricky Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was sent to the canvas by a left hook directly to the face in the early round, getting up unsteadily to his position on the count of eight.
But the red-colored equipment of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was marked up soon, heavy swelling appearing under his orbital area as the duo exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over 12 rounds.
The victor edged the verdict with scoring tallies of draw, narrow lead and clearer margin, but both fighters were persistent and courageous, exactly what their idol Hatton embodied.
