British light heavyweight Buatsi secured a victory yet unimpressive beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a competitive ten-round bout at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the styles never quite clicked, with frequent clinching marring the action.
The officials judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but how do champions respond? They prevail," Buatsi commented in the arena as he was greeted with boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Various analysts believed Parker, thirty-one, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-bout career.
Buatsi Secures Victory in Dull Encounter
The fighter's advancement has slowed in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a final roll of the dice - under recent support.
He began cautiously, with both competitors willing to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to barely take the better periods.
The crowd's patience diminished because of constant clinching, with Parker going down various instances during grappling.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Inspires All-Manchester Contest
In the secondary fight, Arthur overcame Rea by split-decision in a excellent Manchester vs Manchester contest to secure the continental championship.
The stadium fell silent for a deeply poignant memorial ahead of the contest as ten bells sounded in honor of Manchester's own Ricky Hatton, who died at age 46 in last month.
It was the first major televised match card in the urban center since the passing of Hatton. As the last bell faded, the silence transitioned to a roar of "only one Hatton".
The action inside the ropes matched the emotion. Arthur was floored by a left hook squarely on the chin in the early round, getting up unsteadily to his feet on the referee's count.
But the traditional red attire of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was injured quickly, significant bruising forming under his face as the pair exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the pace hardly diminished over the full distance.
Arthur barely secured the verdict with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were relentless and full of heart, everything their inspiration Hatton stood for.
