UK light-heavyweight Buatsi earned a win yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a close ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful individual punches in a match where the approaches never quite clicked, with frequent clinching spoiling the action.
The scoring judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest showing but what defines champions? They succeed," the victor stated in the ringside as he was greeted with boos from the crowd.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, merited the victory but the British boxer experienced the second defeat of his 28-fight career.
The Champion Claims Win in Slow Encounter
Buatsi's progress has slowed in recent years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers willing to be aggressive in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to edge the more effective stanzas.
The audience's tolerance diminished amid frequent holding, with Parker slipping to the canvas various instances during close exchanges.
The underdog closed well, but many rounds were difficult to score. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the supporting bout, Arthur defeated Rea by divided decision in a terrific all-Manchester contest to win the continental belt.
The venue fell silent for a extremely emotional tribute ahead of the contest as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the location since Hatton's passing. As the last bell faded, the quiet transitioned to a roar of "there's only one Hatton".
The action within the ring equaled the emotion. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the second round, rising shakily to his feet on the eight count.
But the red-colored equipment of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was marked up soon, significant bruising forming under his orbital area as the pair traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, responded in kind with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur barely secured the decision with scoring tallies of draw, close margin and 115-112, but both fighters were persistent and full of heart, all the qualities their inspiration Hatton embodied.
