British light-heavyweight Buatsi earned a win yet disappointing debut to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close ten-round contest at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a fight where the techniques never quite clicked, with constant grabbing marring the action.
Two judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best display but how do champions respond? They succeed," Buatsi stated in the ringside as he was met by boos from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th career victory.
Various analysts thought Parker, 31, should have been awarded the decision but the British boxer suffered the second defeat of his 28-fight professional journey.
Buatsi Earns Victory in Slow Encounter
The fighter's advancement has plateaued in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with both competitors prepared to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to edge the more effective periods.
The spectators' acceptance decreased during constant clinching, with the challenger slipping to the canvas several times during grappling.
The underdog closed well, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent local rivalry fight to secure the continental belt.
The venue grew hushed for a extremely emotional memorial before the fight as 10 bells rang in memory of the city's native Hatton, who passed away 46 years old in last month.
It was the first major televised fight night in the urban center since Hatton's passing. As the last bell disappeared, the stillness transitioned to a chant of "only one Hatton".
The competition inside the ropes 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 referee's count.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was damaged quickly, heavy swelling developing under his eye as the pair exchanged punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the result with official scores of 114-114, narrow lead and wider gap, but both fighters were determined and courageous, exactly what their hero Hatton represented.
