UK light heavyweight Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a close 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a contest where the techniques failed to mesh, with frequent clinching spoiling the fight.
Two judges had it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi commented in the ringside as he was met by disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts thought Parker, 31, merited the decision but the British boxer endured the second loss of his 28-fight career.
Buatsi Secures Triumph in Dull Encounter
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of British boxing, he now stands trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors prepared to take risks in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second bout at light-heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who worked behind a solid defense to barely take the cleaner stanzas.
The spectators' acceptance diminished because of frequent holding, with the challenger going down various instances during close exchanges.
Parker closed well, but multiple periods were challenging to assess. Although, former world champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the presence of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester contest to win the European light heavyweight championship.
The venue fell silent for a profoundly moving memorial ahead of the contest as ten bells rang in remembrance of local hero Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the last bell faded, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The fighting inside the ropes reflected the emotion. Arthur was sent to the canvas by a powerful punch flush on the chin in the early round, rising shakily to his footing on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was damaged quickly, noticeable damage appearing under his orbital area as the pair landed punishing blows.
The challenger, 27, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
The victor narrowly won the verdict with scoring tallies of even score, close margin and wider gap, but each competitor were relentless and brave, exactly what their idol Hatton embodied.
