UK light-heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive 10-round fight at the boxing arena.
But Parker had his moments, landing some telling individual punches in a match where the approaches never quite clicked, with constant grabbing spoiling the fight.
The officials scored it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what defines champions? They prevail," Buatsi said in the ring as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts felt the challenger, 31, deserved the win but the contender suffered the second setback of his 28-fight professional journey.
The Champion Earns Triumph in Slow Encounter
His career development has stalled in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with both competitors willing to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who worked behind a solid defense to edge the more effective stanzas.
The crowd's patience decreased amid constant clinching, with Parker falling to the floor several times during clinches.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor moves on, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific all-Manchester contest to win the European light heavyweight title.
The stadium became quiet for a extremely emotional tribute prior to the bout as 10 bells rang in memory of Manchester's own Hatton, who died aged 46 in recently.
It was the first major televised fight night in the city since Hatton's passing. As the concluding sound ended, the silence transitioned to a roar of "only one Ricky Hatton".
The action during the bout equaled the emotion. Arthur was floored by a powerful punch directly to the chin in the initial stage, standing uncertainly to his position on the eight count.
But the red-colored attire of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was marked up soon, heavy swelling forming under his face as the pair traded punishing blows.
The challenger, 27, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the pace seldom decreased over twelve rounds.
Arthur edged the verdict with scoring tallies of 114-114, narrow lead and wider gap, but both fighters were relentless and courageous, all the qualities their hero the legend represented.
