UK light-heavyweight Buatsi made a winning yet disappointing beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful single shots in a match where the approaches failed to mesh, with frequent clinching disrupting the action.
The scoring judges judged it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," Buatsi said in the arena as he was faced boos from the spectators.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a 20th pro success.
Several boxing experts believed the challenger, thirty-one, deserved the decision but the British boxer suffered the second loss of his 28-fight professional journey.
Buatsi Claims Victory in Dull Encounter
The fighter's advancement has stalled in the past few years through contract problems and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors willing to overcommit in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who utilized a protective stance to edge the better stanzas.
The audience's tolerance diminished during regular grabbing, with the challenger slipping to the canvas various instances during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a excellent local rivalry contest to secure the continental championship.
The arena grew hushed for a profoundly moving homage before the fight as a bell ceremony sounded in memory of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the concluding sound ended, the silence gave way to a chant of "there's only one Ricky Hatton".
The competition within the ring equaled the feeling. Arthur was sent to the canvas by a significant blow squarely on the face in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was damaged quickly, noticeable damage forming under his orbital area as the fighters exchanged hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the action rarely dropped over the full distance.
The victor narrowly won the verdict with judges scorecards of even score, close margin and 115-112, but the two athletes were relentless and brave, exactly what their hero Hatton embodied.
