British light-heavyweight Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful individual punches in a match where the techniques never quite clicked, with frequent clinching marring the contest.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but what defines champions? They win," the victor commented in the ring 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 Callum Smith in February - to claim a 20th career victory.
Several boxing experts thought the challenger, 31, merited the victory but the British boxer suffered the second loss of his 28-fight professional journey.
The Champion Earns Triumph in Slow Encounter
Buatsi's progress has plateaued in recent years through management issues and long spells of lack of fights.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with neither fighter willing to take risks in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to narrowly win the more effective stanzas.
The spectators' acceptance decreased during frequent holding, with Parker slipping to the canvas several times during grappling.
The underdog ended stronger, but multiple periods were difficult to score. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Contest
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a terrific Manchester vs Manchester battle to secure the continental championship.
The arena fell silent for a extremely emotional homage before the fight as ten bells rang in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised boxing event in the location since Hatton's passing. As the last bell disappeared, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The action within the ring matched the feeling. The eventual winner was knocked down by a left hook directly to the jaw in the early round, rising shakily to his footing on the referee's count.
But the traditional red attire of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was marked up soon, significant bruising forming under his orbital area as the pair exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the action seldom decreased over the full distance.
Arthur edged the decision with scoring tallies of even score, 115-113 and clearer margin, but the two athletes were persistent and full of heart, all the qualities their idol Hatton represented.
