UK light-heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a close ten-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful single shots in a contest where the approaches never quite clicked, with excessive holding marring the action.
Two judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest display but how do champions respond? They succeed," Buatsi said in the arena as he was greeted with disapproval from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts believed the challenger, 31, deserved the victory but the British boxer experienced the second setback of his 28-bout career.
The Champion Secures Victory in Uneventful Affair
Buatsi's progress has slowed in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter ready to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a tight guard to narrowly win the cleaner stanzas.
The audience's tolerance diminished during frequent holding, with Parker going down several times during close exchanges.
The underdog closed well, but several stanzas were difficult to score. However, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Russian star Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to win the European light heavyweight championship.
The arena became quiet for a extremely emotional homage ahead of the contest as a bell ceremony rang in honor of local hero Ricky Hatton, who died aged 46 in recently.
It was the first major televised fight night in the urban center since Hatton's passing. As the final chime disappeared, the quiet transitioned to a chant of "there's just one Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was floored by a left hook squarely on the chin in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was damaged soon, significant bruising forming under his eye as the duo exchanged hard punches.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the action hardly diminished over twelve rounds.
Arthur barely secured the verdict with official scores of even score, narrow lead and 115-112, but each competitor were persistent and full of heart, everything their hero the legend stood for.
