UK light heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a fight where the styles didn't complement each other, with excessive holding disrupting the contest.
The officials judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest display but what do champions do? They succeed," the victor commented in the arena as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers thought the challenger, 31, merited the victory but the contender endured the second loss of his 28-match professional journey.
Buatsi Secures Victory in Slow Affair
The fighter's advancement has stalled in recent years through contract problems 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 - perhaps 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 cagey early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second fight at light-heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to narrowly win the cleaner rounds.
The audience's tolerance decreased during constant clinching, with Parker falling to the floor several times during clinches.
The underdog closed well, but several stanzas were challenging to assess. Although, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Arthur overcame Rea by split-decision in a terrific all-Manchester contest to secure the European light heavyweight belt.
The venue fell silent for a deeply poignant homage ahead of the contest as 10 bells tolled in memory of local hero Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the location since Hatton's passing. As the final chime disappeared, the quiet transitioned to a roar of "only one Hatton".
The fighting during the bout equaled the feeling. Arthur was sent to the canvas by a left hook flush on the jaw in the early round, rising shakily to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was marked up soon, significant bruising appearing under his orbital area as the pair traded hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the pace seldom decreased over the full distance.
Arthur barely secured the verdict with official scores of draw, narrow lead and 115-112, but each competitor were persistent and brave, exactly what their inspiration Hatton embodied.
