UK light heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a close ten-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a match where the approaches never quite clicked, with frequent clinching disrupting the fight.
Two judges scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They win," the victor said in the ring as he was met by boos from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts thought Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter endured the second loss of his 28-match professional journey.
The Champion Earns Win in Slow Match
Buatsi's progress has plateaued in the past few years through contract problems and long spells of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to achieve expectations - maybe in what seems like a last chance - under recent support.
He started carefully, with the two boxers prepared to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for Buatsi, who utilized a tight guard to narrowly win the better stanzas.
The crowd's patience decreased during constant clinching, with the challenger slipping to the canvas various instances during clinches.
Parker closed well, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Fuels Local Rivalry Battle
In the supporting bout, Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to capture the continental title.
The arena fell silent for a extremely emotional memorial before the fight as ten bells sounded in remembrance of local hero Ricky Hatton, who died 46 years old in recently.
It was the premier televised fight night in the location since the passing of Hatton. As the final chime faded, the stillness became a roar of "there's only one Ricky Hatton".
The competition inside the ropes equaled the sentiment. Arthur was knocked down by a left hook flush on the face in the early round, rising shakily to his position on the eight count.
But the traditional red attire of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was damaged soon, heavy swelling forming under his eye as the fighters traded powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over twelve rounds.
Arthur barely secured the verdict with judges scorecards of draw, narrow lead and clearer margin, but both fighters were persistent and brave, all the qualities their idol the legend represented.
