British light heavyweight Buatsi made a winning yet disappointing beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a close 10-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a match where the techniques never quite clicked, with constant grabbing disrupting the contest.
The officials judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi said in the ring as he was faced negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th pro success.
Some ringside observers believed the challenger, thirty-one, should have been awarded the decision but the contender endured the second loss of his 28-match career.
Buatsi Earns Triumph in Slow Encounter
The fighter's advancement has plateaued in the past few years through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now stands trying to achieve expectations - perhaps in what seems like a last chance - under recent support.
He started carefully, with the two boxers prepared 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 suffering a hand injury in the contest, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, creating difficulties for Buatsi, who operated with a tight guard to edge the more effective rounds.
The audience's tolerance diminished during regular grabbing, with Parker falling to the floor several times during close exchanges.
The underdog ended stronger, but many rounds were difficult to score. However, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher Benavidez, and Russian champion Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a outstanding local rivalry contest to win the European light-heavyweight championship.
The venue became quiet for a profoundly moving homage before the fight as 10 bells tolled in remembrance of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the location since the passing of Hatton. As the concluding sound ended, the silence gave way to a shout of "there's just one Ricky Hatton".
The fighting during the bout matched the sentiment. Arthur was sent to the canvas by a powerful punch squarely on the chin in the initial stage, standing uncertainly to his feet on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up early, heavy swelling developing under his face as the pair landed punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur barely secured the decision with scoring tallies of even score, 115-113 and clearer margin, but the two athletes were determined and courageous, exactly what their hero Hatton embodied.
