British light-heavyweight Buatsi earned a win yet underwhelming start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a competitive 10-round fight at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a match where the styles didn't complement each other, with frequent clinching marring the action.
The officials had it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my best showing but what do champions do? They succeed," Buatsi said in the arena as he was met by negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Various analysts felt the challenger, 31, should have been awarded the decision but the British boxer suffered the second setback of his 28-bout career.
The Champion Claims Triumph in Dull Match
The fighter's advancement has stalled in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to realize his potential - perhaps in what feels like a last chance - under recent support.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who worked behind a protective stance to barely take the better periods.
The crowd's patience diminished during constant clinching, with Parker slipping to the canvas various instances during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Fight
In the co-main event, Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry battle to win the European light heavyweight championship.
The stadium grew hushed for a extremely emotional homage prior to the bout as 10 bells rang in memory of the city's native Hatton, who died aged 46 in recently.
It was the first major televised fight night in the city since the passing of Hatton. As the last bell disappeared, the silence became a chant of "there's just one Hatton".
The action during the bout matched the sentiment. Arthur was floored by a left hook squarely on the face in the second round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red attire of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was damaged soon, significant bruising forming under his orbital area as the pair landed hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the action hardly diminished over twelve rounds.
The victor barely secured the verdict with official scores of 114-114, 115-113 and wider gap, but the two athletes were relentless and courageous, exactly what their idol Hatton stood for.
