UK light heavyweight Buatsi made a winning yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a contest where the approaches failed to mesh, with constant grabbing spoiling the action.
The scoring judges scored it 96-94 in favor to the winner, with the third a even score.
"It wasn't my greatest performance but what defines champions? They win," Buatsi stated in the ring as he was met by disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts thought the challenger, thirty-one, merited the decision but the contender suffered the second defeat of his 28-match career.
Buatsi Claims Triumph in Uneventful Match
Buatsi's progress has slowed in recent times through contract problems and extended periods of inactivity.
Previously predicted as the future of British boxing, he now stands trying to realize his potential - perhaps in what appears to be a last chance - under recent support.
He began cautiously, with neither fighter prepared to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second fight at the higher weight class.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to barely take the better stanzas.
The crowd's patience decreased because of regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker closed well, but multiple periods were hard to judge. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian champion Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester fight to capture the European light heavyweight belt.
The arena fell silent for a deeply poignant memorial prior to the bout as ten bells rang in remembrance of the city's native Ricky Hatton, who died aged 46 in September.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound faded, the stillness became a chant of "there's just one Hatton".
The action within the ring equaled the emotion. Arthur was knocked down by a powerful punch flush on the jaw in the early round, rising shakily to his feet on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up quickly, noticeable damage forming under his orbital area as the duo traded powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor barely secured the decision with official scores of draw, close margin and wider gap, but both fighters were determined and full of heart, exactly what their inspiration Hatton embodied.
