UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight 10-round fight at the boxing arena.
But Parker found success, delivering effective single shots in a contest where the approaches failed to mesh, with frequent clinching marring the fight.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest display but what defines champions? They succeed," the victor commented 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 thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt the challenger, 31, should have been awarded the victory but the British boxer experienced the second defeat of his 28-match professional journey.
The Champion Earns Triumph in Slow Affair
Buatsi's progress has plateaued in recent times through contract problems and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to realize his potential - maybe in what feels like a last chance - under recent support.
He started carefully, with the two boxers prepared to take risks in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second appearance at light-heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who utilized a protective stance to edge the cleaner stanzas.
The crowd's patience decreased amid frequent holding, with Parker falling to the floor various instances during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. However, ex-champion the boxing analyst - working with Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a terrific Manchester vs Manchester battle to win the continental belt.
The stadium grew hushed for a deeply poignant homage ahead of the contest as ten bells tolled in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the premier televised match card in the city since the passing of Hatton. As the last bell disappeared, the stillness became a shout of "there's just one Hatton".
The action inside the ropes reflected the feeling. The eventual winner was floored by a significant blow flush on the face in the second round, getting up unsteadily to his footing on the eight count.
But the red-colored attire of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was damaged quickly, noticeable damage developing under his face as the fighters landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur edged the decision with scoring tallies of even score, 115-113 and 115-112, but the two athletes were persistent and brave, all the qualities their inspiration Hatton stood for.
