British light heavyweight Buatsi earned a win yet disappointing start to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight ten-round fight at the Manchester venue.
But the challenger found success, delivering effective single shots in a fight where the styles didn't complement each other, with constant grabbing marring the action.
The scoring judges had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest display but what defines champions? They succeed," Buatsi stated in the ringside as he was met by negative reactions from the spectators.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Various analysts believed the challenger, 31, should have been awarded the win but the contender endured the second setback of his 28-bout professional journey.
Buatsi Earns Victory in Uneventful Encounter
The fighter's advancement has plateaued in the past few years through management issues and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under recent support.
He started carefully, with neither fighter ready to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to barely take the more effective rounds.
The crowd's patience diminished during frequent holding, with Parker falling to the floor several times during grappling.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding local rivalry fight to win the European light-heavyweight championship.
The stadium became quiet for a profoundly moving homage before the fight as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the first major televised match card in the location since Hatton's passing. As the final chime disappeared, the silence gave way to a shout of "only one Hatton".
The competition inside the ropes reflected the feeling. Arthur was knocked down by a significant blow directly to the chin in the second round, standing uncertainly to his feet on the eight count.
But the traditional red attire of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was marked up early, noticeable damage developing under his orbital area as the pair traded hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over twelve rounds.
The victor edged the decision with judges scorecards of draw, close margin and wider gap, but both fighters were determined and brave, exactly what their idol the legend embodied.
