British light heavyweight Buatsi made a winning yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective individual punches in a contest where the approaches failed to mesh, with constant grabbing marring the action.
The scoring judges had it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my best display but what defines champions? They prevail," Buatsi stated in the ring as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Several boxing experts believed the challenger, 31, should have been awarded the victory but the contender experienced the second defeat of his 28-match career.
The Champion Earns Triumph in Slow Affair
Buatsi's progress has slowed in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - possibly in what seems like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to be aggressive in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second bout at light heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for the favorite, who operated with a tight guard to narrowly win the better periods.
The spectators' acceptance decreased during regular grabbing, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Motivates All-Manchester Contest
In the supporting bout, Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester battle to win the European light-heavyweight championship.
The venue grew hushed for a extremely emotional memorial prior to the bout as a bell ceremony sounded in memory of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised match card in the urban center since Hatton's passing. As the last bell faded, the stillness became a shout of "there's just one Hatton".
The fighting within the ring equaled the sentiment. Arthur was knocked down by a powerful punch flush on the face in the second round, rising shakily to his footing on the referee's count.
But the Manchester United-red shorts of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was damaged quickly, noticeable damage forming under his eye as the pair exchanged punishing blows.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the pace seldom decreased over the full distance.
The victor edged the result with scoring tallies of 114-114, close margin and 115-112, but both fighters were relentless and brave, exactly what their hero Hatton represented.
