UK light heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight 10-round bout at the Manchester venue.
But the challenger found success, delivering effective clean blows in a match where the styles never quite clicked, with excessive holding disrupting the contest.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my greatest performance but what do champions do? They win," the victor commented in the ringside as he was faced boos from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Various analysts thought the challenger, 31, should have been awarded the win but the contender experienced the second defeat of his 28-fight career.
The Champion Claims Triumph in Dull Encounter
The fighter's advancement has slowed in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - maybe in what seems like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter ready to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who worked behind a protective stance to barely take the better periods.
The spectators' acceptance decreased amid regular grabbing, with the challenger falling to the floor several times during grappling.
The underdog ended stronger, but many rounds were difficult to score. However, former world champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Fuels Local Rivalry Contest
In the co-main event, Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester fight to secure the European light heavyweight title.
The venue grew hushed for a profoundly moving tribute prior to the bout as ten bells tolled in honor of the city's native Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the concluding sound faded, the silence became a shout of "only one Hatton".
The competition within the ring matched the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the chin in the early round, standing uncertainly to his position on the referee's count.
But the red-colored equipment of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was marked up quickly, heavy swelling developing under his orbital area as the duo landed punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur barely secured the result with scoring tallies of even score, narrow lead and 115-112, but the two athletes were relentless and full of heart, everything their hero Hatton embodied.
