British light-heavyweight Buatsi earned a win yet unimpressive debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a contest where the styles never quite clicked, with excessive holding marring the action.
Two judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but what defines champions? They prevail," the victor commented in the ringside as he was faced negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to claim a 20th career victory.
Various analysts felt the challenger, thirty-one, should have been awarded the win but the British boxer suffered the second defeat of his 28-fight career.
The Champion Secures Victory in Dull Affair
Buatsi's progress has slowed in recent times through management issues and considerable time of lack of fights.
Once tipped as the future of British boxing, he now is positioned trying to achieve expectations - maybe in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers prepared to overcommit in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who worked behind a solid defense to barely take the cleaner rounds.
The crowd's patience decreased during frequent holding, with the challenger falling to the floor several times during grappling.
Parker closed well, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Inspires Local Rivalry Fight
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to win the continental title.
The venue grew hushed for a deeply poignant tribute prior to the bout as 10 bells sounded in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound ended, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The action inside the ropes equaled the feeling. Arthur was floored by a powerful punch flush on the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was marked up quickly, noticeable damage developing under his face as the pair exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the decision with judges scorecards of even score, narrow lead and clearer margin, but the two athletes were relentless and courageous, everything their inspiration Hatton stood for.
