British light-heavyweight Buatsi made a winning yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close 10-round contest at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with excessive holding marring the contest.
Two judges scored it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what defines champions? They win," the victor stated in the ringside as he was faced disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, 31, merited the victory but the Derbyshire fighter endured the second defeat of his 28-bout career.
Buatsi Earns Win in Uneventful Encounter
His career development has plateaued in the past few years through management issues and long spells of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers willing to overcommit in a tentative opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to barely take the cleaner periods.
The audience's tolerance diminished amid constant clinching, with Parker slipping to the canvas several times during close exchanges.
Parker closed well, but many rounds were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Homage Fuels Manchester Derby Contest
In the co-main event, Arthur beat Rea by divided decision in a outstanding all-Manchester contest to capture the European light heavyweight title.
The arena became quiet for a profoundly moving memorial ahead of the contest as a bell ceremony sounded in honor of local hero Hatton, who died aged 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the final chime disappeared, the silence transitioned to a chant of "there's only one Ricky Hatton".
The competition inside the ropes reflected the emotion. Arthur was floored by a powerful punch directly to the jaw in the early round, getting up unsteadily to his footing on the eight count.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up quickly, noticeable damage appearing under his eye as the fighters traded powerful shots.
The challenger, 27, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor edged the decision with official scores of even score, narrow lead and 115-112, but both fighters were determined and full of heart, all the qualities their hero Hatton represented.
