UK light heavyweight Buatsi made a winning yet underwhelming start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful individual punches in a fight where the styles failed to mesh, with frequent clinching marring the contest.
The officials had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best display but what defines champions? They prevail," the victor stated in the ring as he was met by boos from the audience.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, merited the decision but the contender endured the second loss of his 28-match professional journey.
The Champion Claims Win in Dull Match
His career development has plateaued in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to realize his potential - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who utilized a protective stance to barely take the cleaner rounds.
The spectators' acceptance decreased because of regular grabbing, with Parker slipping to the canvas various instances during grappling.
The underdog ended stronger, but many rounds were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Hatton Memorial Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester contest to capture the European light-heavyweight title.
The venue became quiet for a deeply poignant tribute prior to the bout as ten bells tolled in memory of local hero Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast match card in the city since Hatton's passing. As the concluding sound ended, the silence transitioned to a shout of "there's just one Ricky Hatton".
The fighting during the bout matched the emotion. The eventual winner was floored by a left hook directly to the jaw in the second round, standing uncertainly to his feet on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was injured quickly, significant bruising developing under his face as the duo landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over twelve rounds.
The victor barely secured the result with official scores of 114-114, close margin and wider gap, but the two athletes were determined and full of heart, all the qualities their inspiration Hatton embodied.
