UK light heavyweight Buatsi secured a victory yet unimpressive debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a close 10-round contest at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a contest where the techniques failed to mesh, with excessive holding disrupting the contest.
The scoring judges had it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what defines champions? They prevail," the victor commented in the arena as he was met by disapproval from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts felt Parker, thirty-one, merited the win but the contender endured the second loss of his 28-fight professional journey.
The Champion Claims Triumph in Slow Encounter
The fighter's advancement has stalled in the past few years through management issues and long spells of lack of fights.
Once tipped as the next star of the British fight scene, he now finds himself trying to achieve expectations - maybe in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors prepared to be aggressive in a cagey opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second bout at light heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a tight guard to barely take the more effective stanzas.
The crowd's patience decreased amid frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. However, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Fuels Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent all-Manchester fight to secure the European light-heavyweight championship.
The stadium fell silent for a deeply poignant homage prior to the bout as 10 bells sounded in memory of the city's native Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the last bell faded, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The action inside the ropes reflected the sentiment. The eventual winner was sent to the canvas by a significant blow squarely on the face in the second round, rising shakily to his feet on the eight count.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up quickly, significant bruising forming under his eye as the duo traded punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the decision with judges scorecards of draw, 115-113 and wider gap, but the two athletes were persistent and full of heart, everything their hero the legend embodied.
