British light-heavyweight Buatsi made a winning yet disappointing beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight 10-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the styles failed to mesh, with frequent clinching spoiling the action.
The scoring judges judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," the victor commented in the arena as he was met by negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Several boxing experts believed Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
Buatsi Claims Victory in Slow Affair
His career development has plateaued in recent years through promotional disputes and considerable time of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers willing to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who operated with a solid defense to narrowly win the cleaner rounds.
The spectators' acceptance wore thin amid frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a excellent local rivalry fight to secure the European light heavyweight title.
The arena fell silent for a deeply poignant memorial before the fight as a bell ceremony tolled in honor of Manchester's own Hatton, who died aged 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the concluding sound ended, the silence transitioned to a shout of "there's only one Ricky Hatton".
The action during the bout equaled the feeling. Arthur was sent to the canvas by a left hook squarely on the jaw in the early round, standing uncertainly to his position on the referee's count.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was marked up soon, heavy swelling appearing under his orbital area as the pair traded powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over the full distance.
Arthur edged the decision with scoring tallies of even score, close margin and wider gap, but each competitor were persistent and courageous, all the qualities their idol the legend stood for.
