UK light-heavyweight Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a competitive ten-round bout at the Manchester venue.
But the challenger found success, landing some telling clean blows in a contest where the approaches didn't complement each other, with constant grabbing disrupting the contest.
Two judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best display but how do champions respond? They win," the victor said in the arena as he was met by disapproval from the audience.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Several boxing experts thought the challenger, 31, deserved the win but the British boxer endured the second loss of his 28-match career.
Buatsi Earns Victory in Uneventful Encounter
Buatsi's progress has slowed in recent times through management issues and long spells of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with both competitors willing to take risks in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for the favorite, who utilized a solid defense to edge the more effective stanzas.
The crowd's patience wore thin during frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the secondary fight, Arthur defeated Rea by split-decision in a outstanding all-Manchester fight to secure the continental title.
The arena fell silent for a deeply poignant homage ahead of the contest as ten bells sounded in memory of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound disappeared, the silence became a chant of "there's just one Hatton".
The fighting within the ring reflected the feeling. The eventual winner was floored by a significant blow squarely on the jaw in the initial stage, rising shakily to his position on the count of eight.
But the traditional red attire of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was injured soon, noticeable damage developing under his eye as the duo landed punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the pace rarely dropped over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of even score, close margin and wider gap, but each competitor were persistent and courageous, exactly what their inspiration the legend stood for.
