UK light heavyweight Buatsi made a winning yet unimpressive start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a fight where the styles failed to mesh, with constant grabbing marring the contest.
The officials judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest performance but what defines champions? They succeed," the victor said in the arena as he was greeted with disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts believed the challenger, 31, merited the victory but the Derbyshire fighter endured the second defeat of his 28-fight professional journey.
Buatsi Secures Victory in Slow Affair
His career development has stalled in recent times through contract problems and extended periods of inactivity.
Once tipped as the future of British boxing, he now stands trying to realize his potential - maybe in what seems like a last chance - under fresh management.
He began cautiously, with the two boxers ready to take risks in a cagey initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was making just his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who worked behind a tight guard to barely take the cleaner periods.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
The underdog closed well, but multiple periods were difficult to score. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Hatton Memorial Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding all-Manchester contest to capture the European light heavyweight title.
The arena became quiet for a profoundly moving homage ahead of the contest as ten bells tolled in memory of local hero Hatton, who succumbed 46 years old in September.
It was the premier televised fight night in the urban center since Hatton's death. As the last bell ended, the stillness became a shout of "there's only one Ricky Hatton".
The fighting during the bout matched the sentiment. Arthur was knocked down by a left hook flush on the chin in the early round, standing uncertainly to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was injured soon, heavy swelling developing under his orbital area as the pair landed hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the verdict with scoring tallies of even score, narrow lead and clearer margin, but both fighters were relentless and brave, everything their inspiration Hatton embodied.
