British light heavyweight Buatsi earned a win yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a close 10-round bout at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a contest where the approaches didn't complement each other, with excessive holding marring the fight.
Two judges judged it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi commented in the ring as he was met by boos from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts thought the challenger, 31, deserved the win but the British boxer experienced the second defeat of his 28-bout career.
Buatsi Secures Victory in Slow Affair
Buatsi's progress has plateaued in recent years through contract problems and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to achieve expectations - possibly in what appears to be a last chance - under fresh management.
He began cautiously, with both competitors willing to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to barely take the more effective rounds.
The spectators' acceptance decreased because of constant clinching, with Parker going down various instances during grappling.
Parker finished the stronger, but multiple periods were challenging to assess. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Contest
In the co-main event, Arthur beat Rea by close verdict in a excellent Manchester vs Manchester battle to win the European light heavyweight championship.
The venue fell silent for a extremely emotional memorial prior to the bout as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the first major televised fight night in the location since Hatton's death. As the final chime disappeared, the silence became a shout of "there's only one Ricky Hatton".
The fighting inside the ropes reflected the sentiment. Arthur was sent to the canvas by a left hook directly to the jaw in the second round, standing uncertainly to his position on the count of eight.
But the traditional red equipment of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was injured soon, significant bruising developing under his face as the pair traded punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the decision with judges scorecards of draw, close margin and clearer margin, but the two athletes were relentless and brave, all the qualities their idol Hatton stood for.
