British light-heavyweight Buatsi earned a win yet underwhelming beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a tight ten-round contest at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a match where the techniques didn't complement each other, with constant grabbing spoiling the fight.
Two judges had it 96-94 to the winner, with the final judge a even score.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi said in the arena as he was met by disapproval from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers believed the challenger, 31, merited the victory but the Derbyshire fighter suffered the second defeat of his 28-match career.
Buatsi Claims Win in Dull Match
The fighter's advancement has slowed in the past few years through promotional disputes and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - maybe in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors ready to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to edge the better rounds.
The crowd's patience diminished amid constant clinching, with the challenger going down several times during grappling.
Parker ended stronger, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Hatton Memorial Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a outstanding local rivalry contest to win the continental championship.
The venue grew hushed for a deeply poignant homage before the fight as ten bells tolled in memory of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound faded, the stillness became a chant of "only one Ricky Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was floored by a powerful punch flush on the jaw in the second round, getting up unsteadily to his position on the count of eight.
But the traditional red attire of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was injured soon, heavy swelling developing under his eye as the fighters exchanged hard punches.
The challenger, 27, landed power punches but Arthur, thirty-four, matched the intensity with effective uppercuts as the pace hardly diminished over the full distance.
Arthur barely secured the verdict with judges scorecards of even score, 115-113 and wider gap, but the two athletes were determined and brave, exactly what their idol Hatton represented.
