UK light heavyweight Buatsi secured a victory yet disappointing beginning to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a tight 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a contest where the approaches didn't complement each other, with excessive holding spoiling the fight.
Two judges had it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi stated in the arena as he was greeted with negative reactions from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts thought Parker, thirty-one, deserved the decision but the Derbyshire fighter endured the second defeat of his 28-fight career.
Buatsi Claims Triumph in Uneventful Match
The fighter's advancement has stalled in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to realize his potential - perhaps in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter ready to be aggressive in a cagey initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after suffering a hand injury mid-fight, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to edge the cleaner rounds.
The audience's tolerance wore thin because of constant clinching, with Parker going down various instances during clinches.
The underdog ended stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Motivates All-Manchester Battle
In the secondary fight, Arthur defeated Rea by divided decision in a outstanding all-Manchester fight to win the European light-heavyweight belt.
The arena became quiet for a profoundly moving memorial prior to the bout as 10 bells sounded in memory of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the final chime ended, the stillness gave way to a chant of "there's just one Ricky Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was sent to the canvas by a significant blow directly to the chin in the early round, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was marked up soon, significant bruising appearing under his face as the pair exchanged punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor barely secured the result with official scores of 114-114, 115-113 and wider gap, but the two athletes were persistent and courageous, everything their inspiration the legend represented.
