UK light heavyweight Buatsi secured a victory yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a contest where the techniques failed to mesh, with frequent clinching disrupting the contest.
Two judges judged it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi commented in the ringside as he was met by boos from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers thought the challenger, 31, should have been awarded the decision but the contender experienced the second loss of his 28-match career.
Buatsi Secures Victory in Dull Match
Buatsi's progress has stalled in recent times through contract problems and long spells of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors ready to take risks in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who utilized a protective stance to narrowly win the better rounds.
The spectators' acceptance decreased because of frequent holding, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but multiple periods were hard to judge. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to secure the continental title.
The venue grew hushed for a extremely emotional memorial ahead of the contest as ten bells sounded in memory of local hero Ricky Hatton, who died aged 46 in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the last bell ended, the quiet became a roar of "there's just one Ricky Hatton".
The action during the bout matched the sentiment. The eventual winner was sent to the canvas by a left hook flush on the face in the second round, rising shakily to his footing on the count of eight.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, the opponent - in opposing blue - was injured early, heavy swelling developing under his face as the duo traded punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the pace hardly diminished over 12 rounds.
Arthur edged the verdict with judges scorecards of draw, 115-113 and clearer margin, but both fighters were relentless and courageous, everything their idol Hatton stood for.
