UK light heavyweight Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a competitive 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful individual punches in a match where the styles didn't complement each other, with constant grabbing spoiling the fight.
Two judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," Buatsi said in the ringside as he was met by boos from the spectators.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts believed the challenger, 31, deserved the decision but the British boxer suffered the second defeat of his 28-match career.
The Champion Secures Triumph in Dull Encounter
His career development has plateaued in recent times through contract problems and extended periods of lack of fights.
Previously predicted as the next star of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter willing to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to edge the better stanzas.
The spectators' acceptance decreased amid frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog closed well, but many rounds were hard to judge. However, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Arthur defeated Rea by split-decision in a excellent all-Manchester fight to secure the continental championship.
The stadium fell silent for a extremely emotional homage before the fight as 10 bells sounded in honor of the city's native Hatton, who passed away aged 46 in last month.
It was the premier televised fight night in the location since Hatton's death. As the final chime faded, the stillness became a chant of "there's only one Hatton".
The action inside the ropes reflected the feeling. Arthur was floored by a significant blow squarely on the jaw in the initial stage, rising shakily to his feet on the eight count.
But the red-colored equipment of the fighter soon came forward again. Across the ring, the opponent - in competitive blue - was injured soon, significant bruising developing under his orbital area as the pair exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the pace hardly diminished over 12 rounds.
The victor edged the decision with scoring tallies of 114-114, narrow lead and 115-112, but the two athletes were determined and full of heart, all the qualities their idol Hatton stood for.
