British light-heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive 10-round fight at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a fight where the styles never quite clicked, with frequent clinching disrupting the action.
Two judges scored it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest showing but what do champions do? They succeed," the victor said in the ringside as he was met by negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Several boxing experts felt the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter experienced the second defeat of his 28-fight professional journey.
Buatsi Secures Win in Slow Affair
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers willing to be aggressive in a calculated initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who utilized a tight guard to narrowly win the better stanzas.
The spectators' acceptance wore thin amid frequent holding, with Parker falling to the floor several times during grappling.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Rea by split-decision in a terrific Manchester vs Manchester battle to capture the European light-heavyweight title.
The stadium became quiet for a extremely emotional tribute prior to the bout as ten bells tolled in remembrance of the city's native Hatton, who succumbed 46 years old in last month.
It was the first major televised boxing event in the location since Hatton's passing. As the final chime disappeared, the quiet became a chant of "there's just one Hatton".
The competition within the ring reflected the sentiment. Arthur was knocked down by a left hook directly to the chin in the second round, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was injured early, significant bruising forming under his face as the duo exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action hardly diminished over twelve rounds.
Arthur narrowly won the result with scoring tallies of even score, close margin and wider gap, but both fighters were persistent and brave, everything their hero Hatton stood for.
