British light heavyweight Buatsi secured a victory yet underwhelming beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective clean blows in a fight where the styles failed to mesh, with constant grabbing disrupting the contest.
The scoring judges had it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my finest performance but how do champions respond? They prevail," the victor said in the ringside as he was faced boos from the crowd.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts believed the challenger, thirty-one, deserved the victory but the Derbyshire fighter endured the second defeat of his 28-fight career.
Buatsi Claims Victory in Dull Encounter
His career development has stalled in recent times through promotional disputes and considerable time of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to achieve expectations - possibly in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers willing to overcommit in a calculated early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who operated with a tight guard to narrowly win the better periods.
The spectators' acceptance decreased because of frequent holding, with the challenger falling to the floor several times during clinches.
Parker finished the stronger, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a excellent all-Manchester fight to capture the European light heavyweight belt.
The venue grew hushed for a deeply poignant tribute ahead of the contest as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who died at age 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the last bell ended, the stillness became a shout of "only one Ricky Hatton".
The fighting inside the ropes reflected the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the second round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was marked up soon, significant bruising appearing under his eye as the duo traded hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor barely secured the verdict with scoring tallies of even score, close margin and 115-112, but the two athletes were determined and brave, everything their inspiration Hatton represented.
