British light heavyweight Buatsi secured a victory yet disappointing start to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight ten-round fight at the boxing arena.
But the challenger had his moments, landing some telling clean blows in a contest where the styles never quite clicked, with frequent clinching marring the fight.
The officials judged it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best showing but what do champions do? They win," the victor commented in the ringside as he was greeted with disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers thought the challenger, 31, merited the decision but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
Buatsi Claims Win in Slow Encounter
His career development has stalled in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a last chance - under new promotional backing.
He opened tentatively, with neither fighter ready to take risks in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to barely take the more effective rounds.
The crowd's patience wore thin because of constant clinching, with Parker falling to the floor multiple occasions during grappling.
Parker ended stronger, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Battle
In the co-main event, Arthur defeated Rea by close verdict in a terrific all-Manchester contest to secure the continental belt.
The arena became quiet for a profoundly moving tribute before the fight as ten bells rang in memory of local hero Ricky Hatton, who died at age 46 in recently.
It was the premier televised match card in the city since Hatton's passing. As the last bell ended, the silence transitioned to a chant of "there's only one Ricky Hatton".
The action during the bout matched the sentiment. The eventual winner was floored by a significant blow squarely on the face in the second round, standing uncertainly to his position on the eight count.
But the traditional red attire of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured early, significant bruising forming under his orbital area as the duo landed punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the action rarely dropped over the full distance.
The victor narrowly won the verdict with official scores of 114-114, 115-113 and wider gap, but both fighters were relentless and courageous, everything their idol Hatton embodied.
