British light heavyweight Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a tight 10-round bout at the boxing arena.
But Parker found success, landing some telling individual punches in a contest where the styles failed to mesh, with constant grabbing marring the action.
The scoring judges scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best showing but how do champions respond? They win," Buatsi said in the ring as he was met by boos from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to secure a 20th career victory.
Some ringside observers felt the challenger, 31, deserved the win but the contender endured the second defeat of his 28-fight professional journey.
The Champion Earns Win in Slow Encounter
Buatsi's progress has plateaued in recent years through contract problems and considerable time of lack of fights.
Once tipped as the future of UK boxing, he now stands trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors willing to take risks in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to barely take the more effective stanzas.
The spectators' acceptance decreased amid regular grabbing, with the challenger falling to the floor various instances during clinches.
The underdog closed well, but several stanzas were difficult to score. Although, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Tribute Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Rea by split-decision in a excellent Manchester vs Manchester battle to secure the European light-heavyweight championship.
The arena grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony sounded in memory of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the concluding sound ended, the quiet transitioned to a shout of "only one Ricky Hatton".
The action within the ring equaled the sentiment. Arthur was sent to the canvas by a powerful punch flush on the face in the second round, rising shakily to his position on the referee's count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was damaged soon, heavy swelling developing under his face as the pair exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the action hardly diminished over 12 rounds.
The victor barely secured the decision with scoring tallies of even score, narrow lead and 115-112, but each competitor were determined and courageous, everything their inspiration the legend represented.
