UK light heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the styles never quite clicked, with excessive holding marring the fight.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but what do champions do? They win," the victor said in the arena as he was greeted with disapproval from the crowd.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Various analysts believed the challenger, thirty-one, merited the decision but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
The Champion Claims Victory in Uneventful Encounter
The fighter's advancement has plateaued in recent years through management issues and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a last chance - under recent support.
He opened tentatively, with the two boxers willing to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was making just his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who utilized a solid defense to edge the better rounds.
The crowd's patience wore thin during frequent holding, with Parker falling to the floor various instances during clinches.
Parker ended stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the co-main event, Arthur overcame Rea by divided decision in a outstanding all-Manchester contest to win the continental title.
The venue became quiet for a profoundly moving memorial before the fight as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the urban center since Hatton's death. As the concluding sound ended, the silence gave way to a shout of "there's just one Ricky Hatton".
The competition within the ring equaled the sentiment. The eventual winner was knocked down by a powerful punch directly to the jaw in the second round, standing uncertainly to his footing on the referee's count.
But the red-colored attire of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was marked up quickly, heavy swelling appearing under his orbital area as the pair exchanged hard punches.
The challenger, 27, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur edged the decision with scoring tallies of 114-114, 115-113 and clearer margin, but both fighters were determined and courageous, all the qualities their hero Hatton stood for.
