UK light-heavyweight Buatsi earned a win yet underwhelming debut to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But Parker had his moments, delivering effective clean blows in a match where the techniques never quite clicked, with constant grabbing marring the fight.
Two judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but how do champions respond? They prevail," the victor stated in the ring as he was greeted with disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts believed Parker, 31, should have been awarded the decision but the contender suffered the second loss of his 28-fight career.
The Champion Claims Win in Uneventful Encounter
The fighter's advancement has stalled in the past few years through management issues and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a last chance - under recent support.
He started carefully, with neither fighter prepared to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to edge the better periods.
The audience's tolerance wore thin because of regular grabbing, with the challenger going down several times during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific local rivalry fight to win the European light heavyweight title.
The arena fell silent for a profoundly moving memorial ahead of the contest as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the concluding sound disappeared, the silence gave way to a chant of "there's just one Hatton".
The fighting within the ring reflected the emotion. The eventual winner was floored by a significant blow flush on the chin in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red shorts of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was marked up soon, significant bruising forming under his face as the duo exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur barely secured the result with scoring tallies of draw, 115-113 and 115-112, but the two athletes were persistent and courageous, all the qualities their hero the legend represented.
