British light heavyweight Buatsi earned a win yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a fight where the approaches failed to mesh, with frequent clinching marring the fight.
The officials had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best showing but what do champions do? They win," the victor stated in the ringside as he was met by disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts felt Parker, 31, deserved the decision but the contender suffered the second defeat of his 28-bout professional journey.
Buatsi Earns Win in Uneventful Encounter
The fighter's advancement has stalled in recent times through contract problems and considerable time of lack of fights.
Once tipped as the rising talent of UK boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors prepared to take risks in a calculated opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who utilized a solid defense to barely take the better periods.
The audience's tolerance wore thin because of constant clinching, with the challenger slipping to the canvas various instances during grappling.
The underdog ended stronger, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Fuels Local Rivalry Contest
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a excellent local rivalry fight to capture the European light-heavyweight title.
The arena fell silent for a deeply poignant homage ahead of the contest as a bell ceremony rang in honor of the city's native Hatton, who died aged 46 in September.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the final chime faded, the quiet gave way to a chant of "there's only one Hatton".
The action during the bout matched 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 position on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was damaged soon, noticeable damage forming under his eye as the fighters landed powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity rarely dropped over 12 rounds.
The victor edged the verdict with judges scorecards of 114-114, narrow lead and clearer margin, but both fighters were persistent and full of heart, all the qualities their hero the legend embodied.
