UK light heavyweight Buatsi secured a victory yet unimpressive start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a match where the techniques didn't complement each other, with constant grabbing marring the action.
The officials judged it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," Buatsi commented in the ring as he was greeted with disapproval from the audience.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts thought the challenger, thirty-one, deserved the decision but the British boxer endured the second defeat of his 28-bout professional journey.
The Champion Claims Win in Uneventful Match
The fighter's advancement has stalled in recent times through promotional disputes and considerable time of lack of fights.
Previously predicted as the next star of UK boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter prepared to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight 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 halfway stage, the underdog had made it competitive, causing problems for Buatsi, who utilized a tight guard to barely take the better periods.
The audience's tolerance diminished amid frequent holding, with Parker going down multiple occasions during clinches.
The underdog finished the stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester fight to capture the European light heavyweight championship.
The arena became quiet for a profoundly moving memorial before the fight as a bell ceremony rang in honor of the city's native Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime faded, the silence gave way to a roar of "only one Hatton".
The fighting within the ring equaled the sentiment. Arthur was knocked down by a left hook flush on the jaw in the second round, rising shakily to his footing on the count of eight.
But the Manchester United-red attire of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was damaged early, noticeable damage appearing under his orbital area as the pair exchanged punishing blows.
The challenger, 27, landed power punches but Arthur, 34, matched the intensity with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the result with judges scorecards of draw, 115-113 and 115-112, but the two athletes were determined and full of heart, exactly what their hero the legend embodied.
