British light-heavyweight Buatsi earned a win yet underwhelming beginning to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a tight 10-round contest at the boxing arena.
But Parker had his moments, landing some telling clean blows in a match where the styles didn't complement each other, with excessive holding marring the contest.
The officials had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my greatest showing but what defines champions? They win," Buatsi stated in the arena as he was met by disapproval from the spectators.
The British fighter got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers felt the challenger, 31, deserved the win but the British boxer endured the second setback of his 28-bout professional journey.
Buatsi Claims Win in Uneventful Affair
His career development has stalled in the past few years through promotional disputes and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under recent support.
He opened tentatively, with both competitors ready to overcommit in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who utilized a protective stance to edge the cleaner rounds.
The crowd's patience diminished because of regular grabbing, with the challenger going down multiple occasions during grappling.
Parker finished the stronger, but multiple periods were hard to judge. However, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Arthur defeated Rea by close verdict in a terrific local rivalry battle to secure the European light-heavyweight title.
The arena became quiet for a deeply poignant homage prior to the bout as a bell ceremony tolled in honor of Manchester's own Hatton, who died aged 46 in September.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the concluding sound ended, the silence became a shout of "there's just one Ricky Hatton".
The action during the bout matched the emotion. The eventual winner was knocked down by a significant blow flush on the jaw in the second round, rising shakily to his position on the count of eight.
But the red-colored attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was marked up early, significant bruising appearing under his eye as the duo traded hard punches.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the intensity rarely dropped over the full distance.
The victor narrowly won the decision with official scores of even score, close margin and 115-112, but both fighters were relentless and courageous, everything their inspiration Hatton represented.
