British light-heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round bout at the boxing arena.
But Parker found success, landing some telling single shots in a fight where the styles didn't complement each other, with frequent clinching spoiling the fight.
Two judges scored it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi stated in the ring as he was met by disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts believed Parker, 31, deserved the victory but the Derbyshire fighter suffered the second setback of his 28-bout career.
The Champion Earns Triumph in Dull Match
His career development 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 is positioned trying to realize his potential - perhaps in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second appearance at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who utilized a protective stance to edge the better rounds.
The audience's tolerance diminished during constant clinching, with the challenger going down several times during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, former world champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a outstanding local rivalry contest to win the continental title.
The arena grew hushed for a extremely emotional tribute before the fight as ten bells rang in memory of Manchester's own Ricky Hatton, who died at age 46 in recently.
It was the first major televised fight night in the location since Hatton's death. As the final chime faded, the stillness became a roar of "there's just one Hatton".
The competition within the ring matched the emotion. The eventual winner was sent to the canvas by a left hook directly to the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was injured soon, heavy swelling appearing under his face as the duo landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor narrowly won the decision with judges scorecards of draw, narrow lead and 115-112, but both fighters were relentless and courageous, everything their idol the legend embodied.
