British light-heavyweight Buatsi secured a victory yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a competitive 10-round bout at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a fight where the styles never quite clicked, with constant grabbing marring the fight.
The officials judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They succeed," Buatsi stated in the arena as he was faced boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to secure a 20th career victory.
Some ringside observers thought the challenger, thirty-one, should have been awarded the win but the British boxer suffered the second defeat of his 28-match career.
Buatsi Secures Triumph in Uneventful Encounter
The fighter's advancement has plateaued in recent times through promotional disputes and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to overcommit in a tentative early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who operated with a solid defense to narrowly win the better rounds.
The audience's tolerance decreased during constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
The underdog closed well, but many rounds were challenging to assess. However, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor moves on, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Hatton Memorial Motivates All-Manchester Fight
In the secondary fight, Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester contest to capture the European light-heavyweight title.
The arena grew hushed for a deeply poignant memorial prior to the bout as ten bells rang in memory of the city's native Hatton, who died 46 years old in September.
It was the initial significant broadcast match card in the city since Hatton's passing. As the last bell faded, the stillness became a shout of "only one Hatton".
The competition within the ring matched the emotion. Arthur was floored by a left hook flush on the face in the early round, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was marked up soon, significant bruising appearing under his face as the duo landed hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, matched the intensity with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur narrowly won the decision with scoring tallies of even score, 115-113 and 115-112, but both fighters were determined and courageous, everything their hero Hatton represented.
