British light heavyweight Buatsi made a winning yet disappointing beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful single shots in a match where the approaches didn't complement each other, with frequent clinching disrupting the contest.
Two judges had it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my best showing but what do champions do? They prevail," Buatsi said in the ring as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Various analysts believed Parker, 31, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-match career.
Buatsi Secures Triumph in Slow Match
His career development has slowed in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors ready to take risks in a tentative opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a solid defense to narrowly win the better periods.
The audience's tolerance wore thin amid constant clinching, with the challenger slipping to the canvas various instances during clinches.
Parker ended stronger, but many rounds were difficult to score. However, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the caliber of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific all-Manchester battle to secure the European light heavyweight title.
The venue became quiet for a extremely emotional homage before the fight as a bell ceremony rang in honor of the city's native Hatton, who died at age 46 in recently.
It was the first major televised match card in the urban center since Hatton's death. As the final chime ended, the silence gave way to a roar of "there's only one Ricky Hatton".
The action within the ring matched the feeling. Arthur was floored by a left hook squarely on the chin in the early round, rising shakily to his footing on the referee's count.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured soon, significant bruising developing under his eye as the fighters exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur edged the result with official scores of 114-114, 115-113 and wider gap, but both fighters were determined and brave, everything their inspiration the legend embodied.
