British light heavyweight Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close ten-round bout at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a match where the approaches failed to mesh, with excessive holding marring the fight.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but what defines champions? They succeed," Buatsi said in the ringside as he was faced boos from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts thought Parker, thirty-one, merited the victory but the contender suffered the second setback of his 28-fight professional journey.
The Champion Earns Triumph in Slow Match
Buatsi's progress has plateaued in the past few years through management issues and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers ready to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who utilized a solid defense to narrowly win the better periods.
The crowd's patience wore thin during constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of hard hitter David Benavidez, and Russian champion Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Homage Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a terrific local rivalry fight to capture the European light heavyweight title.
The stadium grew hushed for a deeply poignant memorial prior to the bout as ten bells rang in honor of the city's native Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the last bell ended, the quiet became a shout of "there's just one Ricky Hatton".
The competition during the bout reflected the feeling. Arthur was floored by a powerful punch flush on the chin in the initial stage, standing uncertainly to his footing on the referee's count.
But the red-colored attire of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was injured soon, noticeable damage developing under his eye as the duo traded powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over the full distance.
Arthur narrowly won the decision with official scores of even score, narrow lead and clearer margin, but both fighters were persistent and full of heart, exactly what their inspiration Hatton embodied.
