UK light-heavyweight Buatsi made a winning yet unimpressive start to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close 10-round contest at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a contest where the styles never quite clicked, with frequent clinching marring the contest.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They prevail," the victor said in the ringside as he was met by disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Various analysts felt Parker, thirty-one, deserved the win but the contender endured the second loss of his 28-bout professional journey.
The Champion Claims Victory in Dull Match
Buatsi's progress has plateaued in recent years through promotional disputes and considerable time of lack of fights.
Once tipped as the future of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was competing in only his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who worked behind a protective stance to barely take the better periods.
The spectators' acceptance diminished during regular grabbing, with the challenger going down multiple occasions during clinches.
Parker ended stronger, but several stanzas were challenging to assess. Although, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian champion Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a terrific all-Manchester fight to secure the continental title.
The stadium fell silent for a deeply poignant homage prior to the bout as a bell ceremony tolled in remembrance of local hero Hatton, who died at age 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the concluding sound ended, the stillness gave way to a shout of "there's only one Hatton".
The competition during the bout matched the emotion. Arthur was knocked down by a powerful punch directly to the face in the early round, standing uncertainly to his position on the eight count.
But the red-colored attire of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was injured quickly, noticeable damage appearing under his eye as the pair exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over 12 rounds.
Arthur barely secured the decision with scoring tallies of draw, close margin and 115-112, but the two athletes were determined and brave, exactly what their hero Hatton embodied.
