UK light heavyweight Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a match where the techniques never quite clicked, with constant grabbing marring the fight.
The scoring judges scored it 96-94 to Buatsi, with the third a even score.
"It wasn't my best display but what defines champions? They win," the victor said in the ring as he was met by boos from the audience.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers felt Parker, 31, merited the decision but the contender endured the second loss of his 28-fight professional journey.
The Champion Secures Victory in Slow Affair
His career development has slowed in the past few years through contract problems and extended periods of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors willing to be aggressive in a cagey 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 during the bout, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who utilized a protective stance to barely take the better rounds.
The audience's tolerance wore thin amid frequent holding, with Parker slipping to the canvas several times during clinches.
Parker ended stronger, but many rounds were challenging to assess. However, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the likes of power puncher David Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Inspires Manchester Derby Fight
In the secondary fight, Arthur defeated Rea by close verdict in a excellent local rivalry contest to capture the continental championship.
The stadium grew hushed for a extremely emotional tribute ahead of the contest as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the city since Hatton's death. As the last bell ended, the quiet transitioned to a roar of "there's just one Hatton".
The fighting within the ring matched the emotion. Arthur was floored by a powerful punch directly to the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the traditional red equipment of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was marked up soon, significant bruising forming under his face as the duo exchanged powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the action rarely dropped over twelve rounds.
The victor edged the decision with scoring tallies of draw, 115-113 and clearer margin, but each competitor were relentless and courageous, everything their inspiration the legend embodied.
