British light heavyweight Buatsi made a winning yet unimpressive debut to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close 10-round contest at the Manchester venue.
But the challenger had his moments, delivering effective individual punches in a fight where the styles didn't complement each other, with frequent clinching marring the contest.
The officials had it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best performance but what do champions do? They prevail," Buatsi commented in the ring as he was faced boos from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Various analysts felt Parker, 31, deserved the victory but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
Buatsi Earns Triumph in Slow Affair
His career development has stalled in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He began cautiously, with both competitors willing to take risks in a calculated initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second fight at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a tight guard to barely take the cleaner rounds.
The audience's tolerance decreased during frequent holding, with the challenger slipping to the canvas several times during clinches.
Parker closed well, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor advances, but with the likes of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Inspires Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Rea by split-decision in a terrific Manchester vs Manchester fight to capture the continental title.
The arena grew hushed for a deeply poignant memorial before the fight as 10 bells tolled in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised match card in the location since Hatton's death. As the concluding sound faded, the stillness became a roar of "there's only one Hatton".
The competition during the bout matched the feeling. Arthur was floored by a left hook directly to the face in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was damaged early, significant bruising forming under his face as the duo traded punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over twelve rounds.
The victor barely secured the verdict with scoring tallies of 114-114, narrow lead and wider gap, but both fighters were relentless and full of heart, exactly what their inspiration the legend represented.
