UK light heavyweight Buatsi made a winning yet disappointing debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a contest where the approaches didn't complement each other, with excessive holding marring the contest.
The officials had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest showing but what defines champions? They prevail," Buatsi said in the ring as he was faced boos from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the decision but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
Buatsi Secures Triumph in Uneventful Affair
His career development has slowed in the past few years through management issues and considerable time of inactivity.
Once tipped as the future of UK boxing, he now finds himself trying to achieve expectations - maybe in what seems like a final roll of the dice - under recent support.
He began cautiously, with the two boxers ready to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who utilized a protective stance to barely take the better stanzas.
The crowd's patience diminished because of regular grabbing, with the challenger falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. Although, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor advances, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Memorial Motivates Manchester Derby Battle
In the supporting bout, Arthur beat Rea by split-decision in a terrific all-Manchester contest to win the European light heavyweight championship.
The venue grew hushed for a deeply poignant memorial ahead of the contest as 10 bells tolled in remembrance of Manchester's own Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the concluding sound ended, the silence became a chant of "only one Hatton".
The action within the ring matched the sentiment. Arthur was floored by a left hook flush on the chin in the early round, getting up unsteadily to his footing on the eight count.
But the red-colored attire of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was injured soon, significant bruising forming under his eye as the fighters landed powerful shots.
Rea, 27, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the action hardly diminished over the full distance.
The victor edged the result with judges scorecards of even score, narrow lead and 115-112, but each competitor were persistent and courageous, everything their hero Hatton represented.
