UK light heavyweight Buatsi made a winning yet underwhelming start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a competitive 10-round bout at the Manchester venue.
But Parker found success, delivering effective individual punches in a fight where the approaches never quite clicked, with excessive holding disrupting the action.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," the victor stated in the ringside as he was faced boos from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers believed Parker, 31, should have been awarded the victory but the contender endured the second defeat of his 28-match professional journey.
The Champion Claims Victory in Slow Affair
Buatsi's progress has slowed in recent times through management issues and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to achieve expectations - possibly in what feels like a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to be aggressive in a cagey initial rounds.
Parker, whose only previous defeat 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 light heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who utilized a protective stance to narrowly win the more effective rounds.
The crowd's patience diminished during regular grabbing, with Parker going down various instances during grappling.
Parker closed well, but several stanzas were difficult to score. Although, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Fuels All-Manchester Fight
In the co-main event, Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester battle to capture the European light-heavyweight title.
The venue fell silent for a profoundly moving tribute ahead of the contest as 10 bells rang in memory of Manchester's own Ricky Hatton, who passed away 46 years old in September.
It was the initial significant broadcast fight night in the location since Hatton's death. As the last bell disappeared, the silence gave way to a roar of "there's just one Ricky Hatton".
The competition inside the ropes equaled the sentiment. Arthur was knocked down by a powerful punch squarely on the chin in the initial stage, standing uncertainly to his feet on the eight count.
But the traditional red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured soon, heavy swelling appearing under his eye as the duo landed powerful shots.
Rea, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
The victor narrowly won the result with scoring tallies of draw, 115-113 and 115-112, but each competitor were determined and full of heart, exactly what their hero Hatton embodied.
