UK light-heavyweight Buatsi made a winning yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight 10-round contest at the boxing arena.
But Parker had his moments, landing some telling individual punches in a match where the techniques didn't complement each other, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest performance but what do champions do? They succeed," the victor stated in the arena as he was faced boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers felt Parker, thirty-one, deserved the victory but the contender endured the second defeat of his 28-match professional journey.
Buatsi Earns Win in Dull Affair
The fighter's advancement has plateaued in recent times through promotional disputes and extended periods of lack of fights.
Once tipped as the rising talent of UK boxing, he now is positioned trying to achieve expectations - maybe in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers willing to be aggressive in a tentative initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who worked behind a tight guard to edge the better rounds.
The spectators' acceptance diminished because of frequent holding, with Parker going down various instances during close exchanges.
The underdog closed well, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry fight to capture the European light-heavyweight belt.
The arena became quiet for a deeply poignant memorial ahead of the contest as a bell ceremony sounded in memory of the city's native Hatton, who succumbed at age 46 in last month.
It was the premier televised fight night in the urban center since Hatton's passing. As the last bell faded, the stillness transitioned to a roar of "only one Hatton".
The action during the bout reflected the feeling. The eventual winner was sent to the canvas by a left hook flush on the face in the initial stage, standing uncertainly to his footing on the count of eight.
But the Manchester United-red shorts of the fighter soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was injured early, heavy swelling appearing under his eye as the duo traded hard punches.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over the full distance.
The victor narrowly won the verdict with judges scorecards of even score, close margin and 115-112, but both fighters were relentless and courageous, all the qualities their idol Hatton represented.
