British light heavyweight Buatsi made a winning yet underwhelming beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a fight where the techniques didn't complement each other, with frequent clinching marring the contest.
The officials had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest display but what defines champions? They succeed," the victor said in the ring as he was met by disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Several boxing experts thought Parker, thirty-one, merited the decision but the contender experienced the second defeat of his 28-bout career.
The Champion Claims Win in Slow Encounter
His career development has plateaued in recent years through promotional disputes and considerable time of lack of fights.
Once tipped as the future of British boxing, he now stands trying to realize his potential - possibly in what seems like a last chance - under recent support.
He began cautiously, with the two boxers prepared to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a solid defense to edge the cleaner stanzas.
The spectators' acceptance wore thin because of constant clinching, with Parker slipping to the canvas various instances during grappling.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the secondary fight, Arthur overcame Rea by split-decision in a outstanding local rivalry fight to capture the European light heavyweight belt.
The stadium grew hushed for a extremely emotional homage before the fight as ten bells tolled in memory of the city's native Ricky Hatton, who died at age 46 in recently.
It was the first major televised match card in the city since the passing of Hatton. As the concluding sound disappeared, the quiet transitioned to a shout of "there's just one Hatton".
The fighting within the ring reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the second round, standing uncertainly to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was damaged quickly, significant bruising forming under his face as the pair traded punishing blows.
The challenger, 27, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the pace hardly diminished over the full distance.
Arthur narrowly won the verdict with judges scorecards of draw, narrow lead and clearer margin, but both fighters were relentless and courageous, all the qualities their hero the legend represented.
