UK light-heavyweight Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a close ten-round contest at the Co-op Live Arena.
But Parker found success, connecting with powerful individual punches in a contest where the techniques failed to mesh, with excessive holding marring the contest.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They prevail," the victor commented in the ringside as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts felt Parker, 31, deserved the win but the contender endured the second defeat of his 28-bout career.
The Champion Claims Triumph in Uneventful Affair
His career development has slowed in recent times through promotional disputes and considerable time of lack of fights.
Previously predicted as the future of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter willing to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who worked behind a solid defense to edge the better stanzas.
The crowd's patience wore thin during constant clinching, with Parker falling to the floor several times during clinches.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Motivates All-Manchester Fight
In the co-main event, Arthur defeated Rea by split-decision in a excellent local rivalry contest to capture the European light-heavyweight title.
The arena fell silent for a extremely emotional tribute ahead of the contest as ten bells rang in honor of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound disappeared, the stillness gave way to a chant of "there's just one Hatton".
The action inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a left hook flush on the chin in the second round, standing uncertainly to his footing on the count of eight.
But the red-colored equipment of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was injured quickly, significant bruising appearing under his face as the duo exchanged hard punches.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur narrowly won the verdict with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were relentless and courageous, everything their idol the legend represented.
