UK light-heavyweight Buatsi secured a victory yet underwhelming start to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a fight where the techniques didn't complement each other, with constant grabbing spoiling the contest.
The officials scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but what defines champions? They win," Buatsi said in the ring as he was faced negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th career victory.
Several boxing experts felt the challenger, 31, merited the win but the contender endured the second setback of his 28-bout professional journey.
Buatsi Earns Victory in Slow Match
Buatsi's progress has plateaued in recent years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors willing to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was making just his second fight at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a solid defense to edge the cleaner rounds.
The audience's tolerance wore thin because of constant clinching, with Parker slipping to the canvas various instances during clinches.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent local rivalry battle to secure the European light-heavyweight title.
The stadium became quiet for a deeply poignant memorial ahead of the contest as 10 bells rang in honor of the city's native Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the last bell disappeared, the quiet gave way to a chant of "only one Hatton".
The action within the ring reflected the emotion. Arthur was floored by a left hook flush on the face in the initial stage, getting up unsteadily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was damaged quickly, heavy swelling forming under his face as the pair exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the pace hardly diminished over the full distance.
The victor barely secured the result with scoring tallies of draw, 115-113 and wider gap, but the two athletes were determined and courageous, everything their inspiration the legend stood for.
