British light-heavyweight Buatsi earned a win yet unimpressive start to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a tight ten-round contest at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a fight where the approaches failed to mesh, with constant grabbing disrupting the fight.
The officials had it 96-94 to Buatsi, with the third a even score.
"It wasn't my finest display but how do champions respond? They win," the victor commented in the ring as he was met by boos from the crowd.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Various analysts felt the challenger, 31, deserved the decision but the Derbyshire fighter suffered the second defeat of his 28-fight career.
The Champion Claims Triumph in Uneventful Encounter
Buatsi's progress has plateaued in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now stands trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He began cautiously, with both competitors ready to take risks in a tentative early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was participating in his second appearance at light-heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who operated with a protective stance to edge the more effective stanzas.
The spectators' acceptance wore thin because of regular grabbing, with the challenger going down various instances during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. However, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor advances, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to capture the continental belt.
The stadium grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the final chime faded, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The action during the bout reflected the sentiment. Arthur was sent to the canvas by a powerful punch directly to the face in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up soon, heavy swelling appearing under his eye as the pair exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur edged the verdict with official scores of 114-114, narrow lead and 115-112, but both fighters were determined and brave, exactly what their hero Hatton stood for.
