UK light-heavyweight Buatsi earned a win yet underwhelming beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a competitive ten-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a contest where the techniques failed to mesh, with constant grabbing marring the fight.
The officials had it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my best showing but how do champions respond? They win," the victor said in the ringside as he was met by disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought Parker, 31, should have been awarded the decision but the contender endured the second defeat of his 28-match professional journey.
The Champion Claims Win in Uneventful Match
The fighter's advancement has stalled in the past few years through management issues and extended periods of lack of fights.
Once tipped as the future of British boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand during the bout, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who worked behind a solid defense to barely take the cleaner periods.
The audience's tolerance decreased because of regular grabbing, with Parker going down multiple occasions during close exchanges.
Parker closed well, but multiple periods were hard to judge. Nevertheless, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Homage Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester contest to win the continental title.
The arena fell silent for a extremely emotional tribute prior to the bout as a bell ceremony sounded in remembrance of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the urban center since Hatton's death. As the last bell ended, the stillness became a roar of "there's just one Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the early round, rising shakily to his position on the count of eight.
But the red-colored attire of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was damaged soon, noticeable damage appearing under his face as the duo traded powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with powerful upward punches as the intensity seldom decreased over the full distance.
The victor narrowly won the verdict with judges scorecards of even score, close margin and 115-112, but both fighters were persistent and full of heart, exactly what their hero Hatton represented.
