British light-heavyweight Buatsi earned a win yet underwhelming start to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive 10-round contest at the Manchester venue.
But the challenger had his moments, landing some telling single shots in a contest where the styles failed to mesh, with frequent clinching disrupting the contest.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best showing but how do champions respond? They prevail," the victor commented in the arena as he was met by disapproval from the audience.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts thought Parker, 31, deserved the win but the British boxer endured the second defeat of his 28-bout career.
The Champion Claims Triumph in Uneventful Encounter
The fighter's advancement has slowed in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to achieve expectations - maybe in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers willing to take risks in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second fight at light-heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who operated with a protective stance to narrowly win the more effective periods.
The audience's tolerance wore thin during frequent holding, with the challenger falling to the floor several times during clinches.
Parker finished the stronger, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just one round.
The victor moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the co-main event, Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to capture the European light-heavyweight title.
The stadium became quiet for a deeply poignant homage ahead of the contest as 10 bells rang in honor of Manchester's own Hatton, who succumbed 46 years old in last month.
It was the premier televised match card in the location since Hatton's death. As the last bell ended, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The competition during the bout reflected the feeling. Arthur was sent to the canvas by a left hook flush on the chin in the second round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red attire of the fighter soon came forward again. Across the ring, the opponent - in opposing blue - was marked up early, noticeable damage appearing under his face as the fighters landed punishing blows.
The challenger, 27, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over twelve rounds.
The victor barely secured the decision with judges scorecards of 114-114, close margin and 115-112, but each competitor were determined and full of heart, exactly what their idol Hatton stood for.
