UK light heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling single shots in a match where the techniques never quite clicked, with frequent clinching marring the action.
The scoring judges had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what defines champions? They prevail," Buatsi stated in the arena as he was greeted with disapproval from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Some ringside observers felt the challenger, 31, merited the win but the Derbyshire fighter suffered the second defeat of his 28-match professional journey.
Buatsi Secures Victory in Dull Affair
His career development has stalled in the past few years through management issues and extended periods of lack of fights.
Once tipped as the future of the British fight scene, he now stands trying to achieve expectations - possibly in what appears to be a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second appearance at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a protective stance to narrowly win the better stanzas.
The spectators' acceptance diminished during regular grabbing, with Parker slipping to the canvas several times during clinches.
Parker ended stronger, but many rounds were challenging to assess. However, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Hatton Homage Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a terrific Manchester vs Manchester fight to win the European light-heavyweight title.
The stadium fell silent for a extremely emotional homage ahead of the contest as a bell ceremony tolled in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the last bell faded, the silence became a shout of "there's just one Ricky Hatton".
The fighting during the bout equaled the emotion. Arthur was floored by a powerful punch flush on the jaw in the initial stage, getting up unsteadily to his feet on the eight count.
But the Manchester United-red shorts of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged soon, noticeable damage appearing under his eye as the pair traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur edged the result with official scores of even score, narrow lead and wider gap, but each competitor were relentless and courageous, exactly what their inspiration the legend represented.
