British light heavyweight Buatsi earned a win yet disappointing start to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a contest where the techniques failed to mesh, with excessive holding spoiling the contest.
Two judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what do champions do? They succeed," the victor stated in the ringside as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts thought Parker, thirty-one, deserved the victory but the British boxer experienced the second defeat of his 28-bout professional journey.
The Champion Secures Victory in Slow Affair
Buatsi's progress has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a ultimate opportunity - under recent support.
He started carefully, with the two boxers prepared 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 in the contest, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a solid defense to barely take the more effective stanzas.
The crowd's patience decreased amid constant clinching, with Parker slipping to the canvas several times during grappling.
Parker closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Motivates Manchester Derby Fight
In the supporting bout, Arthur beat Rea by close verdict in a excellent all-Manchester contest to capture the European light-heavyweight championship.
The stadium fell silent for a profoundly moving memorial prior to the bout as ten bells sounded in memory of local hero Hatton, who passed away aged 46 in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound faded, the quiet became a roar of "there's only one Ricky Hatton".
The fighting during the bout equaled the feeling. Arthur was knocked down by a powerful punch squarely on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was damaged soon, heavy swelling appearing under his face as the pair traded hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor barely secured the verdict with official scores of even score, narrow lead and clearer margin, but each competitor were persistent and brave, all the qualities their inspiration the legend represented.
