British light-heavyweight Buatsi made a winning yet unimpressive start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive ten-round bout at the Manchester venue.
But Parker found success, landing some telling clean blows in a fight where the styles failed to mesh, with excessive holding disrupting the action.
The scoring judges judged it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best showing but how do champions respond? They win," the victor stated in the ring as he was faced boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers thought Parker, thirty-one, deserved the victory but the British boxer experienced the second defeat of his 28-fight career.
Buatsi Claims Triumph in Uneventful Affair
The fighter's advancement has stalled in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to be aggressive in a tentative initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who worked behind a protective stance to barely take the more effective periods.
The spectators' acceptance diminished during regular grabbing, with Parker going down various instances during grappling.
Parker ended stronger, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to secure the European light heavyweight championship.
The arena grew hushed for a profoundly moving tribute prior to the bout as 10 bells sounded in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast match card in the city since Hatton's passing. As the final chime disappeared, the silence transitioned to a shout of "there's just one Hatton".
The fighting inside the ropes equaled the feeling. The eventual winner was knocked down by a left hook squarely on the jaw in the initial stage, rising shakily to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was marked up quickly, noticeable damage appearing under his orbital area as the fighters exchanged powerful shots.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur edged the decision with scoring tallies of draw, narrow lead and wider gap, but both fighters were persistent and full of heart, all the qualities their idol Hatton represented.
