UK light heavyweight Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a tight ten-round contest at the Co-op Live Arena.
But Parker had his moments, connecting with powerful single shots in a contest where the styles never quite clicked, with excessive holding spoiling the fight.
The officials judged it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi stated in the ringside as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, merited the victory but the British boxer experienced the second defeat of his 28-match professional journey.
Buatsi Earns Triumph in Dull Match
Buatsi's progress has stalled in recent years through management issues and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors willing to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to barely take the better rounds.
The audience's tolerance wore thin amid regular grabbing, with the challenger falling to the floor several times during close exchanges.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a terrific Manchester vs Manchester contest to win the European light-heavyweight championship.
The venue fell silent for a deeply poignant homage before the fight as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the last bell disappeared, the silence became a shout of "there's just one Ricky Hatton".
The fighting during the bout equaled the emotion. Arthur was knocked down by a powerful punch squarely on the jaw in the second round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red equipment of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was injured early, significant bruising developing under his eye as the fighters exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the result with official scores of 114-114, narrow lead and wider gap, but both fighters were determined and courageous, all the qualities their inspiration Hatton embodied.
