British light heavyweight Buatsi secured a victory yet disappointing debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a close ten-round bout at the Manchester venue.
But the challenger found success, delivering effective clean blows in a match where the styles didn't complement each other, with constant grabbing disrupting the contest.
Two judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what defines champions? They win," the victor said in the ring as he was faced negative reactions from the crowd.
The British fighter returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Various analysts thought Parker, 31, deserved the decision but the British boxer experienced the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Uneventful Encounter
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to realize his potential - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors ready to overcommit in a calculated early stages.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who utilized a tight guard to edge the cleaner rounds.
The audience's tolerance diminished amid frequent holding, with the challenger slipping to the canvas multiple occasions during grappling.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Memorial Motivates Manchester Derby Fight
In the secondary fight, Arthur defeated Rea by close verdict in a outstanding all-Manchester fight to win the continental championship.
The arena grew hushed for a profoundly moving tribute before the fight as a bell ceremony rang in honor of local hero Ricky Hatton, who died 46 years old in last month.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the concluding sound disappeared, the stillness gave way to a chant of "there's only one Ricky Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was sent to the canvas by a significant blow squarely on the jaw in the initial stage, rising shakily to his position on the count of eight.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was injured soon, significant bruising appearing under his eye as the pair landed powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur edged the decision with judges scorecards of even score, close margin and 115-112, but each competitor were relentless and brave, all the qualities their idol the legend stood for.
