British light-heavyweight Buatsi made a winning yet underwhelming debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round fight at the boxing arena.
But the challenger had his moments, connecting with powerful clean blows in a fight where the styles didn't complement each other, with frequent clinching disrupting the fight.
The officials judged it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest showing but what do champions do? They win," Buatsi stated in the arena as he was greeted with boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Several boxing experts thought Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter endured the second loss of his 28-bout career.
The Champion Earns Win in Uneventful Affair
The fighter's advancement has stalled in recent years through management issues and considerable time of lack of fights.
Previously predicted as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who worked behind a solid defense to edge the cleaner stanzas.
The crowd's patience diminished during constant clinching, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a excellent local rivalry battle to secure the European light-heavyweight belt.
The venue became quiet for a deeply poignant homage before the fight as a bell ceremony sounded in remembrance of Manchester's own Hatton, who succumbed aged 46 in September.
It was the premier televised fight night in the city since Hatton's death. As the concluding sound faded, the silence transitioned to a shout of "only one Ricky Hatton".
The competition within the ring reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored attire of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was damaged soon, significant bruising forming under his eye as the duo exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the verdict with scoring tallies of even score, 115-113 and clearer margin, but both fighters were relentless and courageous, exactly what their idol the legend represented.
