British light heavyweight Buatsi earned a win yet underwhelming start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a competitive 10-round contest at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a fight where the styles failed to mesh, with excessive holding marring the action.
The officials judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They win," Buatsi commented in the arena as he was met by disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Some ringside observers felt the challenger, thirty-one, deserved the win but the British boxer suffered the second defeat of his 28-fight career.
Buatsi Earns Win in Slow Affair
Buatsi's progress has plateaued in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors prepared to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who utilized a tight guard to edge the cleaner stanzas.
The crowd's patience diminished during regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Tribute Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry fight to capture the European light-heavyweight title.
The venue fell silent for a profoundly moving tribute ahead of the contest as 10 bells sounded in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised boxing event in the urban center since Hatton's death. As the last bell faded, the stillness gave way to a roar of "there's only one Ricky Hatton".
The action inside the ropes matched the emotion. The eventual winner was floored by a significant blow squarely on the jaw in the initial stage, rising shakily to his position on the count of eight.
But the red-colored attire of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was damaged soon, significant bruising developing under his orbital area as the duo landed punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over 12 rounds.
The victor barely secured the result with scoring tallies of draw, 115-113 and clearer margin, but each competitor were persistent and full of heart, everything their idol the legend embodied.
