UK light heavyweight Buatsi made a winning yet underwhelming beginning to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a tight 10-round contest at the Manchester venue.
But Parker found success, delivering effective individual punches in a fight where the styles failed to mesh, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They win," the victor stated in the arena as he was faced negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers felt the challenger, thirty-one, merited the decision but the British boxer endured the second setback of his 28-bout career.
Buatsi Earns Triumph in Uneventful Encounter
The fighter's advancement has stalled in recent years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under recent support.
He began cautiously, with the two boxers prepared to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a tight guard to barely take the cleaner stanzas.
The audience's tolerance wore thin because of regular grabbing, with the challenger going down multiple occasions during grappling.
Parker ended stronger, but several stanzas were difficult to score. However, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor advances, but with the likes of power puncher David Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates Local Rivalry Fight
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a terrific Manchester vs Manchester contest to secure the continental belt.
The stadium became quiet for a profoundly moving memorial ahead of the contest as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who died at age 46 in last month.
It was the first major televised fight night in the location since the passing of Hatton. As the last bell disappeared, the quiet gave way to a chant of "there's just one Hatton".
The fighting within the ring matched the feeling. Arthur was knocked down by a powerful punch squarely on the jaw in the initial stage, rising shakily to his footing on the count of eight.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was injured early, heavy swelling appearing under his eye as the fighters exchanged hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur narrowly won the result with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were persistent and courageous, everything their idol Hatton embodied.
