UK light heavyweight Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a majority decision in a tight ten-round fight at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the approaches never quite clicked, with excessive holding marring the contest.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but how do champions respond? They win," the victor stated in the arena as he was met by boos from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Some ringside observers believed Parker, 31, merited the win but the British boxer suffered the second defeat of his 28-match professional journey.
Buatsi Earns Triumph in Dull Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under recent support.
He started carefully, with neither fighter prepared to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who operated with a protective stance to barely take the better stanzas.
The crowd's patience decreased amid frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog closed well, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor moves on, but with the caliber of power puncher Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Hatton Homage Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a terrific local rivalry battle to secure the continental title.
The arena grew hushed for a profoundly moving homage before the fight as 10 bells sounded in memory of local hero Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime ended, the silence became a roar of "there's only one Hatton".
The competition during the bout reflected the emotion. The eventual winner was sent to the canvas by a left hook squarely on the chin in the initial stage, getting up unsteadily to his footing on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was marked up soon, significant bruising developing under his face as the pair exchanged powerful shots.
The challenger, 27, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur edged the verdict with scoring tallies of draw, narrow lead and 115-112, but the two athletes were relentless and brave, everything their idol Hatton stood for.
