British light-heavyweight Buatsi secured a victory yet unimpressive start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a close 10-round fight at the boxing arena.
But Parker found success, connecting with powerful single shots in a fight where the approaches didn't complement each other, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best display but how do champions respond? They win," Buatsi commented in the arena as he was met by boos from the spectators.
The Londoner got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts felt Parker, 31, should have been awarded the win but the contender endured the second setback of his 28-bout career.
Buatsi Claims Victory in Slow Affair
His career development has slowed in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with both competitors ready to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was competing in only his second bout at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who utilized a solid defense to barely take the better stanzas.
The crowd's patience decreased because of constant clinching, with Parker slipping to the canvas several times during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent local rivalry contest to win the European light-heavyweight championship.
The venue grew hushed for a profoundly moving tribute ahead of the contest as ten bells sounded in memory of Manchester's own Ricky Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the last bell ended, the quiet became a chant of "there's only one Ricky Hatton".
The action inside the ropes reflected the emotion. Arthur was sent to the canvas by a left hook flush on the face in the second round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was damaged soon, heavy swelling appearing under his eye as the fighters landed powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, fought fire with fire with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the result with official scores of even score, close margin and wider gap, but the two athletes were persistent and brave, everything their hero Hatton embodied.
