British light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round fight at the Manchester venue.
But the challenger had his moments, delivering effective individual punches in a fight where the techniques didn't complement each other, with excessive holding disrupting the fight.
The officials judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest showing but what do champions do? They succeed," Buatsi commented in the ringside as he was met by negative reactions from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Various analysts felt the challenger, 31, merited the victory but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
Buatsi Earns Win in Slow Encounter
Buatsi's progress has slowed in the past few years through promotional disputes and considerable time of lack of fights.
Previously predicted as the future of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers willing to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who operated with a solid defense to barely take the cleaner stanzas.
The crowd's patience wore thin because of regular grabbing, with Parker going down various instances during clinches.
Parker closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the co-main event, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to win the European light-heavyweight belt.
The arena grew hushed for a extremely emotional tribute ahead of the contest as 10 bells sounded in honor of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised match card in the urban center since Hatton's death. As the final chime disappeared, the quiet gave way to a roar of "there's just one Hatton".
The competition during the bout matched the sentiment. The eventual winner was floored by a powerful punch directly to the jaw in the second round, getting up unsteadily to his footing on the referee's count.
But the traditional red shorts of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was injured quickly, heavy swelling appearing under his face as the fighters traded hard punches.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor barely secured the result with judges scorecards of 114-114, narrow lead and clearer margin, but both fighters were determined and courageous, everything their inspiration Hatton represented.
