British light-heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight ten-round fight at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a contest where the approaches didn't complement each other, with frequent clinching marring the action.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best performance but what defines champions? They prevail," Buatsi stated in the ringside as he was greeted with negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the victory but the British boxer endured the second loss of his 28-bout professional journey.
The Champion Earns Win in Uneventful Encounter
The fighter's advancement has plateaued in recent times through contract problems and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to realize his potential - maybe in what seems like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter prepared to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who utilized a solid defense to narrowly win the better rounds.
The crowd's patience decreased during frequent holding, with Parker falling to the floor various instances during grappling.
The underdog closed well, but many rounds were challenging to assess. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to secure the continental belt.
The stadium fell silent for a profoundly moving homage prior to the bout as a bell ceremony tolled in honor of the city's native Ricky Hatton, who passed away 46 years old in September.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime ended, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The action during the bout matched the feeling. Arthur was sent to the canvas by a powerful punch flush on the chin in the initial stage, getting up unsteadily to his footing on the referee's count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was injured early, noticeable damage appearing under his face as the fighters exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur barely secured the result with official scores of 114-114, narrow lead and 115-112, but the two athletes were determined and courageous, all the qualities their inspiration the legend represented.
