UK light-heavyweight Buatsi made a winning yet underwhelming beginning to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling individual punches in a contest where the styles never quite clicked, with excessive holding spoiling the contest.
The scoring judges had it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my best showing but what defines champions? They prevail," Buatsi stated in the arena as he was met by boos from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers believed the challenger, 31, merited the decision but the British boxer experienced the second loss of his 28-match career.
Buatsi Claims Win in Slow Affair
His career development has plateaued in recent times through management issues and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to achieve expectations - possibly in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers ready to overcommit in a tentative opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after breaking his hand mid-fight, was making just his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who worked behind a solid defense to barely take the better periods.
The crowd's patience wore thin during regular grabbing, with Parker falling to the floor multiple occasions during clinches.
The underdog closed well, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the presence of power puncher David Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a terrific local rivalry contest to capture the European light-heavyweight title.
The arena fell silent for a deeply poignant tribute ahead of the contest as ten bells sounded in memory of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the last bell disappeared, the stillness gave way to a chant of "there's only one Hatton".
The competition during the bout reflected the feeling. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the second round, rising shakily to his footing on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was damaged early, significant bruising appearing under his eye as the duo traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the pace seldom decreased over 12 rounds.
The victor edged the decision with official scores of 114-114, close margin and 115-112, but each competitor were persistent and courageous, everything their inspiration Hatton represented.
