UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a majority decision in a close 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful individual punches in a contest where the techniques failed to mesh, with frequent clinching disrupting the fight.
The officials judged it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They prevail," the victor commented in the ringside as he was greeted with boos from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to claim a 20th pro success.
Several boxing experts felt Parker, 31, merited the decision but the contender suffered the second setback of his 28-match professional journey.
Buatsi Earns Win in Uneventful Match
His career development has plateaued in recent years through promotional disputes and long spells of inactivity.
Previously predicted as the future of the British fight scene, he now stands trying to achieve expectations - possibly in what feels like a ultimate opportunity - under recent support.
He started carefully, with the two boxers ready to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who utilized a tight guard to narrowly win the more effective rounds.
The crowd's patience wore thin because of regular grabbing, with Parker falling to the floor multiple occasions during clinches.
Parker closed well, but many rounds were challenging to assess. Although, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a excellent all-Manchester contest to secure the European light heavyweight title.
The venue fell silent for a extremely emotional memorial prior to the bout as 10 bells rang in honor of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised fight night in the city since Hatton's passing. As the last bell faded, the silence gave way to a roar of "only one Hatton".
The fighting during the bout matched the sentiment. Arthur was floored by a significant blow squarely on the chin in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was injured soon, significant bruising appearing under his eye as the fighters traded hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity seldom decreased over 12 rounds.
The victor edged the result with official scores of even score, narrow lead and 115-112, but the two athletes were determined and full of heart, exactly what their inspiration Hatton represented.
