UK light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close ten-round fight at the boxing arena.
But the challenger landed significant shots, landing some telling clean blows in a fight where the styles failed to mesh, with constant grabbing spoiling the contest.
Two judges scored it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what do champions do? They prevail," Buatsi said in the ringside as he was greeted with disapproval from the audience.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts thought Parker, thirty-one, merited the victory but the British boxer endured the second setback of his 28-fight professional journey.
Buatsi Earns Win in Uneventful Affair
The fighter's advancement has stalled in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the future 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 both competitors ready to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a tight guard to narrowly win the better periods.
The crowd's patience diminished during frequent holding, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but multiple periods were difficult to score. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Fight
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a terrific all-Manchester fight to win the European light heavyweight title.
The stadium became quiet for a extremely emotional tribute before the fight as 10 bells sounded in honor of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the final chime ended, the silence transitioned to a shout of "only one Hatton".
The competition within the ring equaled the sentiment. Arthur was knocked down by a powerful punch directly to the jaw in the second round, rising shakily to his footing on the count of eight.
But the Manchester United-red shorts of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was marked up quickly, noticeable damage forming under his face as the pair exchanged hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the pace seldom decreased over the full distance.
The victor narrowly won the verdict with scoring tallies of 114-114, narrow lead and wider gap, but each competitor were persistent and full of heart, everything their hero Hatton represented.
