UK light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a tight 10-round bout at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a match where the styles never quite clicked, with constant grabbing spoiling the contest.
Two judges had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," the victor said in the ringside as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Various analysts felt Parker, 31, deserved the decision but the Derbyshire fighter endured the second defeat of his 28-match professional journey.
Buatsi Earns Win in Slow Encounter
His career development has slowed in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under recent support.
He started carefully, with the two boxers ready to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who utilized a tight guard to narrowly win the better periods.
The crowd's patience decreased amid frequent holding, with Parker falling to the floor various instances during grappling.
The underdog ended stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Hatton Tribute Inspires Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester battle to win the European light-heavyweight belt.
The arena grew hushed for a profoundly moving homage ahead of the contest as a bell ceremony tolled in remembrance of Manchester's own Hatton, who passed away 46 years old in September.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the concluding sound ended, the quiet became a roar of "there's just one Ricky Hatton".
The competition during the bout equaled the feeling. The eventual winner was knocked down by a powerful punch squarely on the jaw in the initial stage, getting up unsteadily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was injured quickly, significant bruising forming under his face as the duo exchanged hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the action seldom decreased over twelve rounds.
The victor narrowly won the decision with judges scorecards of even score, 115-113 and clearer margin, but each competitor were persistent and courageous, exactly what their idol Hatton stood for.
