UK light-heavyweight Buatsi earned a win yet unimpressive debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a close 10-round contest at the Manchester venue.
But Parker found success, connecting with powerful single shots in a fight where the techniques never quite clicked, with constant grabbing disrupting the contest.
Two judges had it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what do champions do? They prevail," Buatsi commented in the ringside as he was greeted with boos from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, 31, deserved the victory but the British boxer experienced the second defeat of his 28-bout professional journey.
The Champion Claims Victory in Uneventful Match
Buatsi's progress has stalled in recent years through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - perhaps in what appears to be a last chance - under recent support.
He opened tentatively, with both competitors ready to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second bout at light-heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who operated with a tight guard to narrowly win the cleaner rounds.
The crowd's patience wore thin because of frequent holding, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to secure the European light-heavyweight belt.
The venue fell silent for a deeply poignant memorial before the fight as a bell ceremony tolled in memory of local hero Ricky Hatton, who passed away at age 46 in last month.
It was the first major televised boxing event in the location since Hatton's passing. As the last bell ended, the quiet gave way to a shout of "there's just one Ricky Hatton".
The fighting within the ring reflected the feeling. The eventual winner was knocked down by a significant blow squarely on the jaw in the early round, getting up unsteadily to his footing on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged soon, heavy swelling appearing under his face as the fighters traded punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur narrowly won the decision with judges scorecards of draw, 115-113 and clearer margin, but both fighters were determined and brave, exactly what their hero the legend represented.
