UK light heavyweight Buatsi made a winning yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a competitive ten-round fight at the Manchester venue.
But the challenger had his moments, delivering effective single shots in a fight where the styles failed to mesh, with excessive holding marring the action.
The scoring judges had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best display but what defines champions? They win," the victor said in the ringside as he was faced negative reactions from the crowd.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Some ringside observers thought the challenger, 31, merited the decision but the British boxer suffered the second loss of his 28-bout career.
The Champion Earns Triumph in Dull Encounter
Buatsi's progress has plateaued in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now is positioned trying to realize his potential - perhaps in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers ready to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was participating in his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who operated with a solid defense to narrowly win the better stanzas.
The crowd's patience diminished amid constant clinching, with Parker slipping to the canvas several times during grappling.
The underdog closed well, but many rounds were hard to judge. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Contest
In the supporting bout, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to secure the European light heavyweight championship.
The stadium fell silent for a extremely emotional homage prior to the bout as ten bells tolled in memory of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the premier televised match card in the urban center since Hatton's passing. As the final chime faded, the quiet gave way to a chant of "only one Hatton".
The competition during the bout reflected the feeling. Arthur was knocked down by a powerful punch flush on the chin in the early round, standing uncertainly to his footing on the referee's count.
But the red-colored attire of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was marked up early, noticeable damage forming under his orbital area as the fighters landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the pace hardly diminished over 12 rounds.
The victor narrowly won the verdict with official scores of 114-114, 115-113 and wider gap, but the two athletes were relentless and full of heart, all the qualities their hero Hatton represented.
