UK light-heavyweight Buatsi secured a victory yet unimpressive start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close ten-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a match where the approaches didn't complement each other, with frequent clinching disrupting the contest.
Two judges scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what defines champions? They prevail," the victor commented in the ringside as he was met by negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Various analysts thought the challenger, thirty-one, merited the victory but the contender suffered the second defeat of his 28-match professional journey.
The Champion Earns Win in Slow Match
His career development has plateaued in the past few years through management issues and long spells of lack of fights.
Previously predicted as the future of British boxing, he now stands trying to realize his potential - perhaps in what seems like a last chance - under recent support.
He opened tentatively, with the two boxers willing to take risks in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who operated with a solid defense to barely take the more effective stanzas.
The spectators' acceptance wore thin during regular grabbing, with the challenger slipping to the canvas multiple occasions during grappling.
Parker closed well, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a excellent local rivalry contest to secure the continental championship.
The venue grew hushed for a profoundly moving homage ahead of the contest as 10 bells rang in honor of the city's native Ricky Hatton, who succumbed aged 46 in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the last bell disappeared, the silence gave way to a roar of "there's just one Hatton".
The fighting inside the ropes reflected the sentiment. Arthur was floored by a significant blow flush on the chin in the early round, getting up unsteadily to his footing on the eight count.
But the red-colored equipment of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was marked up quickly, significant bruising developing under his face as the fighters traded hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur edged the result with judges scorecards of 114-114, close margin and 115-112, but the two athletes were persistent and courageous, exactly what their idol the legend embodied.
