UK light heavyweight Buatsi secured a victory yet unimpressive start to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a competitive 10-round contest at the boxing arena.
But the challenger found success, delivering effective clean blows in a contest where the approaches failed to mesh, with frequent clinching spoiling the fight.
Two judges judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi said in the ringside as he was greeted with boos from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, should have been awarded the decision but the contender experienced the second loss of his 28-bout professional journey.
The Champion Claims Victory in Dull Match
The fighter's advancement has slowed in recent years through contract problems and long spells of inactivity.
Once tipped as the future of UK boxing, he now finds himself trying to realize his potential - possibly in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers ready to take risks in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to narrowly win the more effective stanzas.
The audience's tolerance diminished amid frequent holding, with Parker falling to the floor several times during grappling.
Parker closed well, but several stanzas were difficult to score. However, former world champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Battle
In the co-main event, Arthur defeated Rea by split-decision in a outstanding all-Manchester fight to capture the continental belt.
The stadium grew hushed for a extremely emotional tribute ahead of the contest as a bell ceremony rang in memory of local hero Ricky Hatton, who passed away at age 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's death. As the final chime disappeared, the silence transitioned to a shout of "there's only one Hatton".
The action inside the ropes matched the sentiment. Arthur was floored by a significant blow flush on the chin in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his eye as the fighters landed powerful shots.
The challenger, 27, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur barely secured the decision with official scores of draw, close margin and clearer margin, but the two athletes were relentless and brave, all the qualities their inspiration Hatton represented.
