British light heavyweight Buatsi secured a victory yet underwhelming beginning 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 split decision in a competitive ten-round bout at the boxing arena.
But the challenger had his moments, connecting with powerful single shots in a match where the approaches failed to mesh, with constant grabbing disrupting the fight.
The officials judged it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what do champions do? They prevail," the victor said in the arena as he was greeted with boos from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Various analysts thought the challenger, thirty-one, should have been awarded the victory but the British boxer experienced the second loss of his 28-fight professional journey.
Buatsi Secures Triumph in Slow Encounter
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a last chance - under fresh management.
He began cautiously, with the two boxers willing to overcommit in a tentative opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after breaking his hand during the bout, was participating in his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who operated with a solid defense to barely take the more effective rounds.
The spectators' acceptance decreased during constant clinching, with the challenger going down several times during grappling.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a terrific local rivalry fight to win the European light-heavyweight belt.
The venue became quiet for a profoundly moving tribute prior to the bout as a bell ceremony sounded in honor of local hero Hatton, who succumbed at age 46 in last month.
It was the premier televised fight night in the location since Hatton's death. As the final chime disappeared, the silence transitioned to a chant of "only one Hatton".
The fighting during the bout equaled the feeling. The eventual winner was knocked down by a significant blow squarely on the jaw in the early round, rising shakily to his feet on the eight count.
But the red-colored attire of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was injured early, heavy swelling developing under his orbital area as the fighters exchanged powerful shots.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over 12 rounds.
The victor edged the result with judges scorecards of draw, close margin and clearer margin, but each competitor were relentless and courageous, all the qualities their idol Hatton stood for.
