UK light-heavyweight Buatsi secured a victory yet underwhelming 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 majority decision in a competitive 10-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a match where the techniques never quite clicked, with constant grabbing spoiling the action.
The scoring judges judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest performance but what defines champions? They win," the victor commented in the ring as he was greeted with negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, merited the victory but the contender experienced the second setback of his 28-match career.
The Champion Earns Win in Uneventful Match
Buatsi's progress has plateaued in recent times through management issues and considerable time of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a last chance - under fresh management.
He started carefully, with the two boxers ready to take risks in a cagey initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who worked behind a tight guard to barely take the more effective stanzas.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a excellent local rivalry fight to win the European light heavyweight title.
The arena became quiet for a extremely emotional memorial ahead of the contest as 10 bells rang in honor of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised boxing event in the location since the passing of Hatton. As the last bell ended, the stillness transitioned to a roar of "there's only one Ricky Hatton".
The competition inside the ropes equaled the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the face in the initial stage, standing uncertainly to his feet on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured early, noticeable damage appearing under his face as the duo exchanged hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over 12 rounds.
The victor narrowly won the result with judges scorecards of draw, 115-113 and wider gap, but each competitor were persistent and courageous, exactly what their idol the legend represented.
