UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the styles never quite clicked, with frequent clinching disrupting the action.
The officials had it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what do champions do? They win," the victor said in the ringside as he was faced negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts thought the challenger, 31, deserved the victory but the contender endured the second loss of his 28-fight career.
The Champion Earns Win in Uneventful Match
The fighter's advancement has stalled in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now stands trying to realize his potential - maybe in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors ready to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after suffering a hand injury during the bout, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a tight guard to narrowly win the better rounds.
The spectators' acceptance decreased amid frequent holding, with Parker slipping to the canvas several times during close exchanges.
Parker closed well, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent local rivalry fight to win the continental championship.
The arena fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony rang in remembrance of local hero Hatton, who succumbed at age 46 in last month.
It was the first major televised fight night in the city since Hatton's death. As the final chime disappeared, the silence transitioned to a chant of "there's only one Hatton".
The action inside the ropes equaled the feeling. The eventual winner was floored by a significant blow squarely on the jaw in the early round, getting up unsteadily to his feet on the referee's count.
But the red-colored attire of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was damaged soon, significant bruising appearing under his eye as the duo landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the intensity rarely dropped over twelve rounds.
The victor narrowly won the result with scoring tallies of even score, 115-113 and wider gap, but the two athletes were determined and courageous, everything their hero Hatton stood for.
