UK light-heavyweight Buatsi earned a win yet underwhelming debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive 10-round contest at the boxing arena.
But the challenger had his moments, connecting with powerful single shots in a match where the approaches never quite clicked, with frequent clinching marring the fight.
Two judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but how do champions respond? They prevail," Buatsi stated in the ring as he was met by negative reactions from the crowd.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts felt the challenger, 31, merited the victory but the contender suffered the second loss of his 28-match professional journey.
The Champion Earns Win in Dull Match
The fighter's advancement has stalled in recent times through management issues and long spells of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to realize his potential - possibly in what seems like a last chance - under fresh management.
He opened tentatively, with neither fighter willing to take risks in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who operated with a protective stance to barely take the cleaner stanzas.
The spectators' acceptance diminished amid regular grabbing, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the co-main event, Arthur overcame Rea by close verdict in a outstanding local rivalry contest to win the European light heavyweight belt.
The venue fell silent for a extremely emotional memorial ahead of the contest as 10 bells rang in remembrance of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised match card in the urban center since Hatton's death. As the last bell faded, the silence transitioned to a roar of "there's just one Hatton".
The action inside the ropes reflected the emotion. Arthur was floored by a significant blow flush on the face in the second round, standing uncertainly to his position on the referee's count.
But the red-colored attire of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was damaged quickly, heavy swelling developing under his eye as the fighters traded powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor barely secured the decision with scoring tallies of 114-114, 115-113 and clearer margin, but each competitor were relentless and courageous, all the qualities their idol Hatton stood for.
