UK light heavyweight Buatsi made a winning yet disappointing beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a tight ten-round contest at the Manchester venue.
But the challenger found success, delivering effective clean blows in a match where the techniques never quite clicked, with constant grabbing marring the fight.
Two judges had it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best display but what do champions do? They prevail," Buatsi said in the arena as he was met by boos from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers thought the challenger, 31, deserved the win but the British boxer experienced the second loss of his 28-bout career.
Buatsi Claims Victory in Slow Match
The fighter's advancement has plateaued in the past few years through management issues and considerable time of lack of fights.
Previously predicted as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with both competitors ready to take risks in a calculated early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand in the contest, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who worked behind a tight guard to edge the cleaner periods.
The spectators' acceptance decreased because of constant clinching, with Parker falling to the floor several times during grappling.
The underdog closed well, but several stanzas were difficult to score. However, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Homage Motivates All-Manchester Contest
In the co-main event, Arthur overcame Rea by split-decision in a terrific all-Manchester battle to secure the continental championship.
The venue became quiet for a profoundly moving memorial ahead of the contest as a bell ceremony rang in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the location since Hatton's death. As the final chime ended, the quiet transitioned to a chant of "only one Hatton".
The action within the ring reflected the sentiment. Arthur was sent to the canvas by a left hook flush on the chin in the second round, standing uncertainly to his position on the count of eight.
But the traditional red equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was damaged early, noticeable damage developing under his face as the pair traded punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the pace hardly diminished over 12 rounds.
The victor edged the verdict with scoring tallies of 114-114, close margin and wider gap, but both fighters were relentless and courageous, exactly what their idol the legend represented.
