UK light heavyweight Buatsi secured a victory yet disappointing debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a close ten-round contest at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a contest where the styles didn't complement each other, with excessive holding marring the action.
The officials judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best showing but how do champions respond? They win," the victor commented in the ring as he was met by negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts believed Parker, thirty-one, should have been awarded the decision but the contender suffered the second setback of his 28-match career.
Buatsi Earns Win in Slow Encounter
His career development has plateaued in recent times through management issues and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to realize his potential - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with both competitors willing to be aggressive in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who utilized a solid defense to edge the more effective periods.
The crowd's patience diminished because of regular grabbing, with Parker going down various instances during grappling.
The underdog finished the stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor advances, but with the likes of power puncher David Benavidez, and Russian star Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent all-Manchester contest to capture the continental title.
The stadium fell silent for a profoundly moving memorial ahead of the contest as ten bells tolled in honor of the city's native Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised fight night in the urban center since Hatton's death. As the last bell faded, the quiet gave way to a roar of "only one Hatton".
The action within the ring equaled the feeling. The eventual winner was knocked down by a significant blow flush on the chin in the early round, rising shakily to his feet on the eight count.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was marked up soon, noticeable damage developing under his eye as the pair landed hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the action hardly diminished over twelve rounds.
Arthur barely secured the decision with scoring tallies of even score, 115-113 and wider gap, but the two athletes were relentless and full of heart, everything their hero the legend represented.
