UK light heavyweight Buatsi made a winning yet disappointing start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close ten-round fight at the Manchester venue.
But the challenger found success, delivering effective individual punches in a match where the styles never quite clicked, with constant grabbing marring the fight.
Two judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They prevail," Buatsi commented in the ring as he was faced disapproval from the audience.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Some ringside observers thought Parker, 31, deserved the victory but the British boxer experienced the second setback of his 28-match career.
Buatsi Claims Triumph in Slow Encounter
Buatsi's progress has slowed in recent years through management issues and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second appearance at light heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who worked behind a solid defense to barely take the cleaner periods.
The spectators' acceptance decreased because of regular grabbing, with Parker going down multiple occasions during close exchanges.
Parker closed well, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Hatton Memorial Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a outstanding local rivalry contest to win the continental title.
The stadium grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony rang in honor of Manchester's own Hatton, who died aged 46 in last month.
It was the first major televised fight night in the location since Hatton's death. As the last bell ended, the quiet transitioned to a shout of "there's just one Ricky Hatton".
The action within the ring matched the sentiment. Arthur was knocked down by a significant blow squarely on the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was damaged early, noticeable damage developing under his face as the fighters exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the intensity hardly diminished over 12 rounds.
Arthur edged the verdict with scoring tallies of even score, 115-113 and wider gap, but the two athletes were determined and brave, exactly what their inspiration Hatton stood for.
