UK light-heavyweight Buatsi earned a win yet underwhelming beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective clean blows in a match where the styles didn't complement each other, with excessive holding marring the contest.
The scoring judges scored it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They succeed," Buatsi commented in the arena as he was greeted with disapproval from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Various analysts felt Parker, 31, deserved the decision but the British boxer suffered the second setback of his 28-match professional journey.
The Champion Claims Triumph in Slow Encounter
His career development has slowed in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to realize his potential - maybe in what seems like a ultimate opportunity - under fresh management.
He started carefully, with both competitors willing to be aggressive in a tentative opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who utilized a protective stance to edge the better rounds.
The spectators' acceptance diminished because of frequent holding, with Parker going down various instances during clinches.
The underdog closed well, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Hatton Tribute Motivates Manchester Derby Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding local rivalry fight to secure the continental championship.
The arena grew hushed for a profoundly moving homage prior to the bout as 10 bells tolled in honor of the city's native Ricky Hatton, who died aged 46 in recently.
It was the first major televised match card in the location since Hatton's passing. As the concluding sound ended, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The action during the bout matched the emotion. The eventual winner 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 traditional red attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was injured soon, heavy swelling appearing under his eye as the fighters exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the decision with judges scorecards of draw, 115-113 and clearer margin, but the two athletes were persistent and brave, all the qualities their hero Hatton embodied.
