UK light-heavyweight Buatsi made a winning yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a competitive ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a contest where the approaches failed to mesh, with excessive holding spoiling the fight.
Two judges scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," the victor said in the ringside as he was met by negative reactions from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Various analysts believed Parker, thirty-one, merited the victory but the British boxer endured the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Uneventful Affair
The fighter's advancement has plateaued in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors willing to be aggressive in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who utilized a tight guard to narrowly win the cleaner stanzas.
The audience's tolerance diminished during constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
The underdog finished the stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Memorial Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to secure the continental belt.
The stadium fell silent for a extremely emotional memorial before the fight as 10 bells tolled in honor of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised boxing event in the location since the passing of Hatton. As the last bell disappeared, the silence became a chant of "only one Ricky Hatton".
The action during the bout matched the feeling. Arthur was floored by a powerful punch directly to the jaw in the early round, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was injured soon, noticeable damage developing under his orbital area as the pair exchanged punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over twelve rounds.
Arthur edged the result with scoring tallies of draw, 115-113 and clearer margin, but the two athletes were relentless and courageous, exactly what their inspiration Hatton embodied.
