UK light heavyweight Buatsi made a winning yet unimpressive debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But the challenger had his moments, delivering effective single shots in a contest where the approaches didn't complement each other, with excessive holding spoiling the fight.
The officials judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what do champions do? They win," the victor commented in the arena as he was faced boos from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers thought Parker, 31, should have been awarded the win but the contender endured the second loss of his 28-fight professional journey.
Buatsi Secures Win in Slow Affair
His career development has slowed in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He started carefully, with the two boxers ready to take risks in a calculated opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to narrowly win the more effective periods.
The crowd's patience wore thin because of frequent holding, with Parker falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. However, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a excellent Manchester vs Manchester battle to win the continental championship.
The stadium grew hushed for a profoundly moving memorial before the fight as ten bells tolled in honor of Manchester's own Ricky Hatton, who died at age 46 in recently.
It was the premier televised boxing event in the urban center since Hatton's passing. As the last bell faded, the silence transitioned to a roar of "there's just one Hatton".
The competition during the bout matched the sentiment. Arthur was sent to the canvas by a powerful punch squarely on the chin in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured soon, heavy swelling appearing under his face as the duo exchanged powerful shots.
Rea, 27, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the decision with judges scorecards of 114-114, narrow lead and wider gap, but both fighters were persistent and brave, all the qualities their inspiration Hatton embodied.
