UK light heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a tight 10-round bout at the Manchester venue.
But the challenger had his moments, landing some telling single shots in a match where the approaches failed to mesh, with frequent clinching spoiling the contest.
Two judges had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They win," Buatsi stated in the ringside as he was met by disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts believed Parker, thirty-one, merited the victory but the contender endured the second loss of his 28-fight career.
The Champion Secures Victory in Slow Encounter
Buatsi's progress has slowed in recent years through management issues and considerable time of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a last chance - under fresh management.
He started carefully, with the two boxers ready to be aggressive in a tentative initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who utilized a solid defense to edge the cleaner rounds.
The spectators' acceptance diminished during frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog closed well, but many rounds were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the secondary fight, Arthur defeated Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to win the European light-heavyweight title.
The stadium fell silent for a deeply poignant homage ahead of the contest as 10 bells rang in memory of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the concluding sound disappeared, the silence became a roar of "only one Hatton".
The competition within the ring matched the emotion. Arthur was sent to the canvas by a significant blow squarely on the chin in the initial stage, getting up unsteadily to his position on the referee's count.
But the traditional red equipment of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was damaged soon, noticeable damage appearing under his orbital area as the pair exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor barely secured the verdict with official scores of even score, 115-113 and wider gap, but each competitor were persistent and brave, exactly what their hero the legend embodied.
