UK light heavyweight Buatsi earned a win yet unimpressive beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful individual punches in a match where the techniques failed to mesh, with constant grabbing spoiling the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They win," Buatsi commented in the arena as he was faced boos from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Several boxing experts believed Parker, 31, merited the victory but the British boxer experienced the second setback of his 28-bout career.
Buatsi Secures Triumph in Dull Match
Buatsi's progress has plateaued in recent times through management issues and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a last chance - under new promotional backing.
He began cautiously, with neither fighter prepared to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second fight at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who worked behind a tight guard to narrowly win the more effective rounds.
The audience's tolerance wore thin during constant clinching, with Parker falling to the floor multiple occasions during clinches.
The underdog finished the stronger, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Hatton Memorial Fuels Local Rivalry Battle
In the supporting bout, Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester contest to win the European light heavyweight championship.
The stadium became quiet for a profoundly moving memorial before the fight as ten bells rang in honor of local hero Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound faded, the silence gave way to a roar of "there's just one Hatton".
The action within the ring matched the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the early round, getting up unsteadily to his feet on the eight count.
But the red-colored attire of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was injured early, noticeable damage developing under his eye as the fighters traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur edged the decision with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were relentless and brave, all the qualities their inspiration the legend embodied.
