UK light heavyweight Buatsi earned a win yet disappointing beginning to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive 10-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a match where the styles never quite clicked, with constant grabbing spoiling the fight.
The officials had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my greatest performance but how do champions respond? They succeed," Buatsi commented in the ringside as he was greeted with disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts felt Parker, 31, merited the victory but the contender suffered the second defeat of his 28-fight professional journey.
The Champion Earns Win in Dull Affair
The fighter's advancement has plateaued in recent times through management issues and long spells of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a last chance - under new promotional backing.
He began cautiously, with both competitors willing to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to barely take the better periods.
The crowd's patience diminished amid regular grabbing, with the challenger slipping to the canvas multiple occasions during grappling.
Parker ended stronger, but many rounds were hard to judge. However, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a outstanding local rivalry battle to secure the continental belt.
The stadium grew hushed for a profoundly moving memorial ahead of the contest as 10 bells tolled in honor of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the premier televised match card in the location since Hatton's death. As the last bell faded, the stillness gave way to a chant of "there's only one Ricky Hatton".
The fighting within the ring reflected the feeling. The eventual winner was floored by a powerful punch squarely on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was marked up soon, significant bruising appearing under his orbital area as the pair exchanged powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the decision with judges scorecards of 114-114, close margin and clearer margin, but the two athletes were relentless and courageous, all the qualities their idol the legend represented.
