British light-heavyweight Buatsi secured a victory yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a close ten-round contest at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a fight where the styles never quite clicked, with excessive holding spoiling the action.
The officials had it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They prevail," Buatsi stated in the ringside as he was met by disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts thought Parker, 31, should have been awarded the victory but the British boxer experienced the second loss of his 28-bout professional journey.
The Champion Claims Win in Uneventful Encounter
Buatsi's progress has slowed in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - maybe in what seems like a final roll of the dice - under recent support.
He started carefully, with both competitors prepared to be aggressive in a cagey early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who operated with a solid defense to narrowly win the cleaner periods.
The audience's tolerance decreased during frequent holding, with Parker going down multiple occasions during grappling.
Parker ended stronger, but several stanzas were challenging to assess. However, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by divided decision in a excellent all-Manchester fight to secure the continental title.
The stadium fell silent for a profoundly moving tribute prior to the bout as 10 bells tolled in honor of Manchester's own Ricky Hatton, who passed away at age 46 in September.
It was the first major televised fight night in the location since Hatton's passing. As the final chime disappeared, the stillness gave way to a chant of "only one Hatton".
The competition during the bout reflected the sentiment. The eventual winner was floored by a significant blow squarely on the chin in the early round, rising shakily to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was damaged early, significant bruising developing under his face as the pair landed punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over 12 rounds.
The victor edged the verdict with judges scorecards of draw, 115-113 and clearer margin, but both fighters were relentless and full of heart, all the qualities their idol Hatton represented.
