UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight 10-round contest at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a fight where the approaches never quite clicked, with excessive holding marring the fight.
Two judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest performance but how do champions respond? They prevail," Buatsi said in the ringside as he was faced disapproval from the audience.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts felt Parker, 31, should have been awarded the win but the British boxer endured the second loss of his 28-match professional journey.
The Champion Claims Triumph in Uneventful Encounter
Buatsi's progress has slowed in recent years through management issues and extended periods of lack of fights.
Previously predicted as the next star of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a last chance - under recent support.
He began cautiously, with neither fighter ready to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury during the bout, was participating in his second fight at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who worked behind a tight guard to narrowly win the better rounds.
The spectators' acceptance decreased because of frequent holding, with Parker falling to the floor several times during clinches.
Parker closed well, but several stanzas were difficult to score. However, former world champion the boxing analyst - working with Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester contest to secure the European light-heavyweight championship.
The stadium became quiet for a extremely emotional homage ahead of the contest as 10 bells rang in honor of Manchester's own Hatton, who died aged 46 in last month.
It was the premier televised fight night in the urban center since the passing of Hatton. As the concluding sound faded, the quiet gave way to a shout of "there's only one Hatton".
The competition within the ring equaled the feeling. Arthur was sent to the canvas by a powerful punch flush on the chin in the early round, getting up unsteadily to his feet on the eight count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was marked up quickly, heavy swelling developing under his eye as the fighters landed hard punches.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over 12 rounds.
The victor edged the verdict with official scores of draw, narrow lead and clearer margin, but both fighters were relentless and full of heart, all the qualities their inspiration Hatton stood for.
