British light heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight 10-round fight at the boxing arena.
But Parker landed significant shots, delivering effective single shots in a fight where the approaches never quite clicked, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my best performance but what do champions do? They prevail," the victor commented in the ring as he was faced negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Various analysts felt Parker, 31, deserved the victory but the contender suffered the second setback of his 28-fight career.
Buatsi Earns Victory in Uneventful Match
Buatsi's progress has stalled in recent years through contract problems and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to achieve expectations - possibly in what seems like a final roll of the dice - under fresh management.
He started carefully, with the two boxers willing to take risks in a calculated initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after suffering a hand injury mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a tight guard to edge the better rounds.
The spectators' acceptance wore thin amid regular grabbing, with the challenger going down various instances during grappling.
Parker ended stronger, but multiple periods were difficult to score. However, former world champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific Manchester vs Manchester contest to capture the European light-heavyweight championship.
The venue became quiet for a extremely emotional tribute prior to the bout as a bell ceremony rang in honor of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast match card in the city since Hatton's death. As the final chime disappeared, the quiet gave way to a shout of "there's only one Hatton".
The competition during the bout reflected the emotion. Arthur was sent to the canvas by a significant blow directly to the face in the second round, rising shakily to his position on the eight count.
But the traditional red shorts of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was injured early, noticeable damage appearing under his face as the fighters landed hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
The victor edged the verdict with scoring tallies of 114-114, close margin and wider gap, but both fighters were persistent and full of heart, all the qualities their inspiration the legend stood for.
