UK light-heavyweight Buatsi earned a win yet underwhelming start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful single shots in a match where the techniques didn't complement each other, with excessive holding marring the fight.
The officials judged it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my best performance but how do champions respond? They prevail," the victor commented in the arena as he was met by negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Various analysts thought the challenger, thirty-one, deserved the victory but the contender endured the second loss of his 28-match professional journey.
The Champion Secures Triumph in Uneventful Match
The fighter's advancement has slowed in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with both competitors ready to take risks in a calculated opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was participating in his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who utilized a tight guard to edge the cleaner rounds.
The crowd's patience decreased during regular grabbing, with the challenger falling to the floor several times during clinches.
The underdog ended stronger, but several stanzas were hard to judge. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Contest
In the co-main event, Arthur overcame Bradley Rea by split-decision in a terrific all-Manchester contest to secure the European light-heavyweight title.
The stadium grew hushed for a deeply poignant tribute before the fight as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who succumbed aged 46 in recently.
It was the first major televised boxing event in the location since the passing of Hatton. As the last bell ended, the silence gave way to a roar of "only one Ricky Hatton".
The fighting within the ring equaled the sentiment. The eventual winner was knocked down by a significant blow squarely on the jaw in the early round, standing uncertainly to his position on the eight count.
But the red-colored equipment of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was injured soon, significant bruising appearing under his face as the duo traded hard punches.
The challenger, 27, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor barely secured the verdict with scoring tallies of 114-114, 115-113 and clearer margin, but the two athletes were relentless and full of heart, everything their inspiration the legend stood for.
