UK light-heavyweight Buatsi secured a victory yet disappointing beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling single shots in a match where the approaches failed to mesh, with excessive holding disrupting the contest.
The scoring judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest display but what defines champions? They succeed," the victor commented in the ring as he was met by boos from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th pro success.
Some ringside observers thought the challenger, 31, merited the decision but the British boxer endured the second setback of his 28-bout professional journey.
The Champion Earns Win in Dull Encounter
The fighter's advancement has plateaued in the past few years through contract problems and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to be aggressive in a cagey opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who operated with a protective stance to narrowly win the better periods.
The crowd's patience decreased because of constant clinching, with Parker going down several times during grappling.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Homage Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester contest to secure the continental championship.
The stadium became quiet for a extremely emotional memorial ahead of the contest as a bell ceremony rang in honor of Manchester's own Hatton, who passed away aged 46 in recently.
It was the first major televised fight night in the urban center since Hatton's passing. As the last bell ended, the silence gave way to a shout of "there's only one Hatton".
The fighting within the ring matched the emotion. The eventual winner was knocked down by a left hook flush on the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red equipment of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was marked up soon, significant bruising appearing under his eye as the duo landed punishing blows.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the intensity hardly diminished over the full distance.
The victor edged the verdict with judges scorecards of draw, 115-113 and 115-112, but both fighters were relentless and brave, exactly what their hero the legend embodied.
