UK light heavyweight Buatsi secured a victory yet unimpressive start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful individual punches in a match where the techniques failed to mesh, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my greatest display but how do champions respond? They succeed," the victor stated in the ringside as he was met by boos from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th pro success.
Some ringside observers felt the challenger, 31, should have been awarded the victory but the contender suffered the second loss of his 28-bout career.
The Champion Claims Triumph in Uneventful Match
The fighter's advancement has stalled in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to realize his potential - maybe in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers willing to overcommit in a calculated initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who operated with a solid defense to edge the cleaner periods.
The spectators' acceptance wore thin during constant clinching, with Parker falling to the floor several times during clinches.
Parker closed well, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Motivates All-Manchester Contest
In the secondary fight, Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to capture the European light heavyweight championship.
The arena became quiet for a deeply poignant homage prior to the bout as ten bells sounded in remembrance of Manchester's own Hatton, who succumbed 46 years old in last month.
It was the first major televised boxing event in the city since Hatton's death. As the concluding sound faded, the quiet transitioned to a roar of "only one Hatton".
The action inside the ropes matched the sentiment. Arthur was sent to the canvas by a left hook directly to the chin in the early round, getting up unsteadily to his feet on the count of eight.
But the traditional red attire of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was injured soon, heavy swelling appearing under his eye as the fighters landed powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over 12 rounds.
Arthur edged the decision with scoring tallies of 114-114, close margin and clearer margin, but both fighters were relentless and courageous, all the qualities their inspiration the legend embodied.
