British light-heavyweight Buatsi made a winning yet disappointing start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful clean blows in a fight where the approaches didn't complement each other, with frequent clinching marring the contest.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They prevail," Buatsi said in the ringside as he was greeted with boos from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the decision but the contender endured the second setback of his 28-match professional journey.
Buatsi Earns Triumph in Slow Encounter
Buatsi's progress has plateaued in the past few years through promotional disputes and considerable time of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - perhaps in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second fight at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who utilized a tight guard to edge the better rounds.
The crowd's patience wore thin during frequent holding, with Parker falling to the floor several times during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester contest to win the continental belt.
The arena became quiet for a extremely emotional homage before the fight as ten bells sounded in memory of the city's native Hatton, who died 46 years old in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the concluding sound disappeared, the silence became a chant of "there's just one Ricky Hatton".
The fighting inside the ropes matched the emotion. Arthur was floored by a left hook flush on the chin in the second round, rising shakily to his feet on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was injured soon, noticeable damage appearing under his orbital area as the pair exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the action rarely dropped over 12 rounds.
The victor barely secured the decision with official scores of 114-114, narrow lead and wider gap, but both fighters were persistent and full of heart, all the qualities their inspiration the legend embodied.
