UK light-heavyweight Buatsi secured a victory yet disappointing beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a competitive ten-round fight at the Manchester venue.
But the challenger found success, connecting with powerful individual punches in a fight where the techniques never quite clicked, with frequent clinching spoiling the fight.
The officials scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi commented in the arena as he was met by negative reactions from the audience.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, deserved the victory but the British boxer experienced the second defeat of his 28-bout professional journey.
The Champion Claims Triumph in Slow Encounter
His career development has slowed in recent years through management issues and extended periods of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - maybe in what seems like a final roll of the dice - under fresh management.
He started carefully, with both competitors willing to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who utilized a protective stance to narrowly win the better periods.
The spectators' acceptance decreased during regular grabbing, with Parker falling to the floor several times during grappling.
Parker closed well, but many rounds were difficult to score. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Hatton Memorial Inspires Manchester Derby Fight
In the secondary fight, Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester contest to win the continental championship.
The venue fell silent for a deeply poignant memorial before the fight as ten bells sounded in memory of local hero Hatton, who passed away aged 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the last bell ended, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The fighting within the ring equaled the feeling. Arthur was sent to the canvas by a powerful punch directly to the jaw in the initial stage, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was injured early, heavy swelling developing under his orbital area as the pair traded punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over the full distance.
Arthur edged the verdict with official scores of 114-114, 115-113 and wider gap, but the two athletes were persistent and full of heart, exactly what their hero the legend embodied.
