UK light heavyweight Buatsi earned a win yet disappointing beginning to his career under new promoters 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 10-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a contest where the styles failed to mesh, with excessive holding spoiling the contest.
Two judges judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi commented in the ringside as he was met by boos from the spectators.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Several boxing experts believed the challenger, 31, merited the decision but the contender suffered the second setback of his 28-fight career.
Buatsi Secures Victory in Uneventful Match
Buatsi's progress has stalled in recent years through management issues and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors prepared to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a tight guard to barely take the more effective periods.
The crowd's patience wore thin because of frequent holding, with the challenger falling to the floor multiple occasions during clinches.
Parker finished the stronger, but multiple periods were challenging to assess. However, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Russian star Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Homage Motivates Manchester Derby Fight
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester battle to secure the European light-heavyweight championship.
The arena grew hushed for a profoundly moving tribute before the fight as a bell ceremony tolled in remembrance of local hero Hatton, who died at age 46 in recently.
It was the first major televised match card in the location since Hatton's passing. As the concluding sound disappeared, the silence gave way to a shout of "there's only one Ricky Hatton".
The competition during the bout matched the emotion. Arthur was floored by a left hook directly to the chin in the second round, standing uncertainly to his footing on the eight count.
But the red-colored attire of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up early, noticeable damage appearing under his orbital area as the pair landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over twelve rounds.
Arthur edged the result with scoring tallies of draw, 115-113 and clearer margin, but each competitor were determined and full of heart, exactly what their hero the legend embodied.
