UK light heavyweight Buatsi secured a victory yet disappointing start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a close 10-round bout at the boxing arena.
But the challenger found success, delivering effective single shots in a fight where the styles never quite clicked, with frequent clinching disrupting the fight.
The scoring judges scored it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but what do champions do? They succeed," Buatsi said in the arena as he was met by boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers believed the challenger, 31, should have been awarded the win but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
The Champion Claims Triumph in Slow Affair
Buatsi's progress has stalled in recent years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to realize his potential - maybe in what appears to be a ultimate opportunity - under recent support.
He started carefully, with the two boxers ready to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who utilized a protective stance to edge the cleaner rounds.
The audience's tolerance wore thin during regular grabbing, with the challenger falling to the floor several times during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian champion Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a outstanding Manchester vs Manchester fight to secure the European light-heavyweight title.
The arena fell silent for a deeply poignant homage prior to the bout as a bell ceremony sounded in honor of Manchester's own Hatton, who died at age 46 in recently.
It was the premier televised match card in the urban center since the passing of Hatton. As the last bell ended, the quiet gave way to a roar of "only one Hatton".
The fighting within the ring equaled the feeling. The eventual winner was sent to the canvas by a significant blow flush on the face in the initial stage, rising shakily to his position on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was injured early, significant bruising forming under his face as the duo landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur narrowly won the verdict with official scores of even score, narrow lead and wider gap, but each competitor were determined and brave, all the qualities their inspiration Hatton embodied.
