UK light heavyweight Buatsi earned a win yet disappointing debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close ten-round fight at the boxing arena.
But the challenger had his moments, connecting with powerful individual punches in a contest where the techniques never quite clicked, with frequent clinching disrupting the action.
Two judges judged it 96-94 for to the winner, with the third a even score.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi said in the ringside as he was faced disapproval from the audience.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Various analysts thought the challenger, 31, should have been awarded the win but the Derbyshire fighter suffered the second defeat of his 28-match professional journey.
The Champion Earns Win in Slow Match
Buatsi's progress has plateaued in recent times through management issues and considerable time of lack of fights.
Previously predicted as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter ready 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 breaking his hand in the contest, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who operated with a protective stance to edge the better periods.
The crowd's patience decreased because of regular grabbing, with the challenger falling to the floor several times during close exchanges.
Parker closed well, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian champion Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the supporting bout, Arthur beat Rea by close verdict in a outstanding Manchester vs Manchester contest to secure the continental belt.
The stadium grew hushed for a deeply poignant tribute ahead of the contest as 10 bells sounded in honor of the city's native Ricky Hatton, who died aged 46 in last month.
It was the first major televised fight night in the location since Hatton's death. As the concluding sound faded, the silence became a roar of "there's just one Ricky Hatton".
The fighting within the ring matched the sentiment. Arthur was sent to the canvas by a powerful punch flush on the face in the early round, getting up unsteadily to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was injured quickly, significant bruising developing under his orbital area as the pair exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of draw, narrow lead and clearer margin, but the two athletes were determined and courageous, exactly what their inspiration Hatton embodied.
