British light heavyweight Buatsi earned a win yet underwhelming debut to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a tight 10-round contest at the Co-op Live Arena.
But Parker found success, connecting with powerful clean blows in a match where the styles didn't complement each other, with constant grabbing disrupting the contest.
The officials scored it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my best showing but what do champions do? They succeed," the victor stated in the ring as he was greeted with disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Several boxing experts believed the challenger, thirty-one, deserved the victory but the British boxer endured the second defeat of his 28-fight career.
Buatsi Earns Triumph in Dull Match
His career development has stalled in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a last chance - under fresh management.
He started carefully, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was competing in only his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who operated with a protective stance to edge the better periods.
The spectators' acceptance decreased amid constant clinching, with the challenger falling to the floor multiple occasions during grappling.
Parker finished the stronger, but multiple periods were challenging to assess. However, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the likes of power puncher David Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester battle to win the European light-heavyweight title.
The stadium fell silent for a deeply poignant memorial ahead of the contest as a bell ceremony rang in memory of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the city since the passing of Hatton. As the final chime disappeared, the silence transitioned to a shout of "only one Ricky Hatton".
The action inside the ropes equaled the feeling. Arthur was knocked down by a significant blow squarely on the chin in the early round, standing uncertainly to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was injured quickly, noticeable damage forming under his eye as the duo traded powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the action hardly diminished over twelve rounds.
The victor narrowly won the verdict with official scores of draw, 115-113 and wider gap, but the two athletes were persistent and full of heart, exactly what their hero Hatton stood for.
