British light-heavyweight Buatsi made a winning yet unimpressive start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a fight where the techniques never quite clicked, with constant grabbing spoiling the fight.
Two judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best performance but what defines champions? They prevail," Buatsi commented in the ringside as he was met by negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Various analysts believed the challenger, thirty-one, deserved the victory but the contender endured the second setback of his 28-bout career.
Buatsi Claims Victory in Uneventful Match
His career development has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to achieve expectations - possibly in what seems like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers ready to be aggressive in a calculated opening few rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who operated with a protective stance to barely take the better stanzas.
The audience's tolerance decreased because of regular grabbing, with the challenger going down multiple occasions during close exchanges.
The underdog closed well, but many rounds were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Hatton Tribute Inspires All-Manchester Contest
In the co-main event, Arthur beat Rea by close verdict in a excellent Manchester vs Manchester contest to secure the continental championship.
The stadium fell silent for a profoundly moving tribute prior to the bout as ten bells tolled in honor of the city's native Ricky Hatton, who passed away aged 46 in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the last bell faded, the quiet transitioned to a roar of "only one Hatton".
The action inside the ropes reflected the sentiment. Arthur was knocked down by a powerful punch directly to the face in the initial stage, getting up unsteadily to his position on the eight count.
But the red-colored shorts of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was marked up quickly, significant bruising appearing under his face as the pair traded hard punches.
Rea, 27, landed power punches but the champion, 34, responded in kind with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur edged the result with judges scorecards of even score, 115-113 and wider gap, but the two athletes were determined and full of heart, all the qualities their idol Hatton stood for.
