British light-heavyweight Buatsi secured a victory yet disappointing start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight 10-round fight at the Manchester venue.
But the challenger found success, connecting with powerful clean blows in a match where the approaches didn't complement each other, with excessive holding spoiling the action.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what do champions do? They win," Buatsi stated in the arena as he was met by disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers believed Parker, thirty-one, merited the victory but the British boxer experienced the second setback of his 28-match professional journey.
Buatsi Secures Victory in Slow Match
Buatsi's progress has stalled in recent years through contract problems and considerable time of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a last chance - under recent support.
He opened tentatively, with neither fighter ready to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who worked behind a protective stance to barely take the better stanzas.
The crowd's patience wore thin because of frequent holding, with Parker going down various instances during clinches.
Parker finished the stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the co-main event, Arthur beat Bradley Rea by split-decision in a outstanding local rivalry fight to win the continental championship.
The stadium fell silent for a extremely emotional homage prior to the bout as 10 bells sounded in remembrance of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime ended, the quiet transitioned to a roar of "there's just one Hatton".
The competition during the bout reflected the feeling. The eventual winner was sent to the canvas by a significant blow directly to the jaw in the second round, standing uncertainly to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up soon, heavy swelling developing under his eye as the fighters traded punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur narrowly won the verdict with official scores of 114-114, narrow lead and clearer margin, but each competitor were relentless and brave, everything their inspiration Hatton embodied.
