UK light-heavyweight Buatsi earned a win yet underwhelming start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a tight 10-round bout at the boxing arena.
But the challenger had his moments, landing some telling individual punches in a match where the techniques didn't complement each other, with frequent clinching spoiling the action.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best performance but how do champions respond? They succeed," the victor commented in the ring as he was faced disapproval from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Various analysts believed Parker, 31, should have been awarded the victory but the British boxer suffered the second setback of his 28-fight career.
The Champion Claims Win in Slow Affair
Buatsi's progress has stalled in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to realize his potential - possibly in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors willing to take risks in a calculated initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a solid defense to narrowly win the better rounds.
The spectators' acceptance decreased because of frequent holding, with the challenger slipping to the canvas several times during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. However, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Arthur overcame Rea by divided decision in a outstanding local rivalry fight to win the European light-heavyweight title.
The stadium fell silent for a profoundly moving tribute ahead of the contest as 10 bells tolled in memory of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised fight night in the urban center since Hatton's passing. As the final chime ended, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the early round, rising shakily to his feet on the referee's count.
But the traditional red attire of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was injured quickly, heavy swelling forming under his eye as the duo exchanged hard punches.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the intensity rarely dropped over twelve rounds.
Arthur edged the decision with official scores of even score, 115-113 and clearer margin, but both fighters were persistent and brave, all the qualities their idol Hatton represented.
