British light heavyweight Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a close ten-round fight at the boxing arena.
But Parker had his moments, delivering effective individual punches in a fight where the styles failed to mesh, with excessive holding disrupting the fight.
Two judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but what defines champions? They win," the victor stated in the ring as he was met by boos from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts believed Parker, thirty-one, deserved the win but the British boxer experienced the second loss of his 28-fight career.
The Champion Secures Victory in Uneventful Match
Buatsi's progress has slowed in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the future of British boxing, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under recent support.
He opened tentatively, with both competitors ready to overcommit in a cagey opening few rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at the higher weight class.
By the middle rounds, the underdog had made it competitive, making life awkward for the favorite, who worked behind a solid defense to barely take the cleaner periods.
The audience's tolerance decreased during regular grabbing, with the challenger slipping to the canvas various instances during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Arthur overcame Rea by split-decision in a excellent Manchester vs Manchester battle to capture the European light-heavyweight belt.
The stadium became quiet for a profoundly moving tribute before the fight as a bell ceremony sounded in memory of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the location since Hatton's death. As the concluding sound ended, the quiet gave way to a roar of "only one Hatton".
The competition inside the ropes matched the feeling. The eventual winner was sent to the canvas by a significant blow flush on the jaw in the second round, standing uncertainly to his footing on the referee's count.
But the red-colored equipment of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was damaged soon, heavy swelling developing under his eye as the fighters traded hard punches.
Rea, 27, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor narrowly won the verdict with official scores of even score, narrow lead and 115-112, but each competitor were determined and brave, all the qualities their hero Hatton stood for.
