UK light heavyweight Buatsi earned a win yet unimpressive start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a contest where the techniques never quite clicked, with constant grabbing disrupting the action.
The officials scored it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best performance but what do champions do? They succeed," Buatsi commented in the ring as he was met by boos from the crowd.
The Londoner got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Some ringside observers believed Parker, thirty-one, merited the decision but the contender suffered the second defeat of his 28-match professional journey.
The Champion Secures Triumph in Dull Match
The fighter's advancement has slowed in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - perhaps in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who operated with a protective stance to edge the cleaner periods.
The spectators' acceptance decreased because of constant clinching, with Parker slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a excellent Manchester vs Manchester battle to win the continental belt.
The venue fell silent for a extremely emotional homage before the fight as a bell ceremony tolled in honor of Manchester's own Hatton, who died at age 46 in last month.
It was the initial significant broadcast match card in the city since Hatton's death. As the last bell faded, the stillness became a chant of "only one Hatton".
The competition within the ring equaled the emotion. Arthur was knocked down by a significant blow squarely on the jaw in the early round, standing uncertainly to his position on the eight count.
But the Manchester United-red attire of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was injured quickly, noticeable damage developing under his face as the duo exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur barely secured the result with judges scorecards of even score, close margin and 115-112, but both fighters were persistent and full of heart, exactly what their hero the legend stood for.
