UK light heavyweight Buatsi made a winning yet disappointing beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a competitive 10-round bout at the boxing arena.
But the challenger found success, connecting with powerful clean blows in a contest where the styles never quite clicked, with frequent clinching marring the fight.
Two judges scored it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They succeed," the victor commented in the ring as he was greeted with disapproval from the audience.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Various analysts felt the challenger, 31, merited the decision but the British boxer suffered the second loss of his 28-match professional journey.
The Champion Secures Triumph in Slow Encounter
Buatsi's progress has plateaued in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - possibly in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter ready to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to barely take the better stanzas.
The crowd's patience decreased amid regular grabbing, with the challenger slipping to the canvas several times during clinches.
The underdog ended stronger, but several stanzas were hard to judge. However, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the likes of heavy hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a excellent local rivalry contest to win the European light heavyweight title.
The stadium fell silent for a profoundly moving tribute before the fight as 10 bells tolled in honor of the city's native Hatton, who succumbed at age 46 in last month.
It was the premier televised boxing event in the location since Hatton's death. As the last bell faded, the quiet gave way to a roar of "there's only one Hatton".
The fighting within the ring matched the emotion. The eventual winner was knocked down by a powerful punch flush on the jaw in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red equipment of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was marked up early, significant bruising appearing under his eye as the fighters landed powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over the full distance.
Arthur narrowly won the decision with scoring tallies of 114-114, narrow lead and wider gap, but each competitor were persistent and brave, everything their idol the legend represented.
