UK light heavyweight Buatsi made a winning yet disappointing beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a competitive ten-round fight at the boxing arena.
But the challenger landed significant shots, landing some telling single shots in a fight where the styles didn't complement each other, with constant grabbing spoiling the action.
The scoring judges had it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi said in the arena as he was greeted with disapproval from the audience.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts felt Parker, thirty-one, merited the decision but the Derbyshire fighter suffered the second setback of his 28-match career.
Buatsi Claims Victory in Slow Encounter
His career development has stalled in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the future of the British fight scene, he now stands trying to achieve expectations - possibly in what appears to be a last chance - under recent support.
He started carefully, with neither fighter willing to overcommit in a cagey initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who utilized a tight guard to barely take the more effective rounds.
The audience's tolerance wore thin amid constant clinching, with Parker going down various instances during clinches.
The underdog closed well, but several stanzas were hard to judge. Although, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Contest
In the co-main event, Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester battle to secure the European light-heavyweight belt.
The stadium fell silent for a extremely emotional memorial prior to the bout as 10 bells rang in remembrance of the city's native Hatton, who died 46 years old in last month.
It was the premier televised match card in the location since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a roar of "there's only one Hatton".
The competition during the bout matched the sentiment. Arthur was floored by a powerful punch squarely on the chin in the early round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red equipment of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was injured soon, noticeable damage appearing under his orbital area as the pair exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the intensity rarely dropped over 12 rounds.
Arthur narrowly won the verdict with scoring tallies of even score, narrow lead and clearer margin, but both fighters were determined and courageous, exactly what their inspiration the legend embodied.
