UK light heavyweight Buatsi made a winning yet underwhelming beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a close ten-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling individual punches in a contest where the approaches failed to mesh, with constant grabbing spoiling the contest.
The scoring judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest display but what do champions do? They succeed," Buatsi commented in the ring as he was met by disapproval from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought the challenger, 31, deserved the decision but the British boxer experienced the second defeat of his 28-match professional journey.
Buatsi Claims Victory in Dull Match
The fighter's advancement has slowed in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers willing to take risks in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who worked behind a tight guard to edge the better rounds.
The audience's tolerance decreased amid frequent holding, with the challenger slipping to the canvas several times during clinches.
Parker closed well, but multiple periods were hard to judge. Although, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor progresses, but with the caliber of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the supporting bout, Arthur defeated Rea by split-decision in a terrific all-Manchester fight to capture the continental title.
The arena grew hushed for a profoundly moving tribute ahead of the contest as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away 46 years old in September.
It was the first major televised boxing event in the location since Hatton's death. As the final chime faded, the stillness gave way to a chant of "there's just one Ricky Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was floored by a significant blow squarely on the jaw in the early round, standing uncertainly to his feet on the eight count.
But the traditional red shorts of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was injured soon, noticeable damage appearing under his face as the duo exchanged punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor narrowly won the result with scoring tallies of even score, close margin and wider gap, but each competitor were relentless and courageous, all the qualities their hero the legend stood for.
