UK light-heavyweight Buatsi made a winning yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a contest where the techniques didn't complement each other, with frequent clinching disrupting the fight.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but how do champions respond? They succeed," Buatsi commented in the arena as he was met by boos from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in February - to secure a 20th career victory.
Several boxing experts believed the challenger, thirty-one, merited the victory but the British boxer endured the second defeat of his 28-match professional journey.
The Champion Earns Victory in Dull Encounter
The fighter's advancement has plateaued in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now is positioned trying to realize his potential - possibly in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers prepared to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who operated with a tight guard to edge the cleaner rounds.
The audience's tolerance diminished during frequent holding, with the challenger going down several times during grappling.
Parker closed well, but multiple periods were difficult to score. However, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent all-Manchester fight to capture the European light-heavyweight championship.
The venue became quiet for a extremely emotional memorial ahead of the contest as ten bells tolled in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast fight night in the city since Hatton's death. As the final chime faded, the stillness gave way to a chant of "there's just one Hatton".
The fighting inside the ropes equaled the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the jaw in the second round, getting up unsteadily to his position on the eight count.
But the traditional red equipment of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was marked up quickly, significant bruising forming under his eye as the duo exchanged punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity rarely dropped over the full distance.
Arthur edged the result with official scores of even score, 115-113 and 115-112, but both fighters were determined and courageous, exactly what their inspiration the legend embodied.
