UK light heavyweight Buatsi made a winning yet unimpressive beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight ten-round contest at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a fight where the approaches failed to mesh, with excessive holding disrupting the fight.
The officials scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi commented in the ringside as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th pro success.
Various analysts thought the challenger, thirty-one, merited the victory but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Dull Encounter
The fighter's advancement has slowed in recent times 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 last chance - under fresh management.
He started carefully, with the two boxers ready to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who utilized a tight guard to narrowly win the better periods.
The crowd's patience wore thin during constant clinching, with Parker falling to the floor various instances during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to secure the continental championship.
The venue became quiet for a deeply poignant homage ahead of the contest as a bell ceremony rang in honor of Manchester's own Hatton, who died at age 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's death. As the concluding sound faded, the quiet became a shout of "there's only one Hatton".
The action during the bout matched the sentiment. Arthur was sent to the canvas by a significant blow directly to the jaw in the initial stage, rising shakily to his feet on the eight count.
But the red-colored attire of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up quickly, significant bruising appearing under his orbital area as the fighters exchanged punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur barely secured the result with judges scorecards of even score, close margin and wider gap, but both fighters were relentless and brave, exactly what their hero Hatton represented.
