UK light-heavyweight Buatsi earned a win yet underwhelming start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger had his moments, landing some telling single shots in a fight where the approaches never quite clicked, with constant grabbing disrupting the fight.
The scoring judges judged it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest display but how do champions respond? They prevail," the victor said in the ringside as he was met by negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Various analysts thought the challenger, thirty-one, merited the decision but the contender suffered the second setback of his 28-bout professional journey.
Buatsi Secures Win in Dull Match
The fighter's advancement has stalled in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors willing to take risks in a tentative opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who worked behind a tight guard to edge the more effective periods.
The audience's tolerance decreased during regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor advances, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a terrific all-Manchester battle to capture the European light heavyweight title.
The arena grew hushed for a deeply poignant memorial ahead of the contest as ten bells tolled in honor of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime ended, the stillness became a shout of "there's only one Ricky Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was floored by a significant blow flush on the chin in the initial stage, rising shakily to his footing on the eight count.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was damaged early, heavy swelling appearing under his orbital area as the duo exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur narrowly won the result with scoring tallies of even score, close margin and wider gap, but both fighters were determined and courageous, exactly what their inspiration the legend embodied.
