British light-heavyweight Buatsi made a winning yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful single shots in a match where the techniques didn't complement each other, with frequent clinching spoiling the contest.
The officials scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They succeed," the victor said in the ring as he was met by disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Various analysts believed Parker, 31, deserved the victory but the British boxer suffered the second defeat of his 28-match career.
The Champion Secures Win in Dull Encounter
His career development has plateaued in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what seems like a last chance - under new promotional backing.
He started carefully, with the two boxers willing to take risks in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who utilized a solid defense to narrowly win the cleaner stanzas.
The spectators' acceptance wore thin because of regular grabbing, with the challenger falling to the floor various instances during close exchanges.
Parker ended stronger, but many rounds were challenging to assess. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a excellent local rivalry fight to secure the European light-heavyweight championship.
The stadium fell silent for a profoundly moving tribute before the fight as a bell ceremony tolled in remembrance of Manchester's own Hatton, who passed away at age 46 in last month.
It was the first major televised boxing event in the city since Hatton's death. As the final chime faded, the stillness became a shout of "only one Ricky Hatton".
The competition during the bout reflected the sentiment. Arthur was knocked down by a left hook squarely on the chin in the initial stage, getting up unsteadily to his footing on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged early, heavy swelling developing under his eye as the fighters landed punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the intensity seldom decreased over 12 rounds.
Arthur barely secured the decision with judges scorecards of even score, close margin and wider gap, but both fighters were relentless and courageous, exactly what their hero the legend represented.
