British light-heavyweight Buatsi made a winning yet unimpressive debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a close 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the styles never quite clicked, with constant grabbing disrupting the contest.
The officials had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best display but what defines champions? They prevail," the victor commented in the ring as he was greeted with disapproval from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Several boxing experts felt Parker, thirty-one, deserved the decision but the British boxer suffered the second setback of his 28-bout professional journey.
Buatsi Earns Victory in Dull Match
His career development has slowed in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under new promotional backing.
He started carefully, with neither fighter willing to be aggressive in a calculated early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who utilized a solid defense to narrowly win the more effective rounds.
The spectators' acceptance decreased during regular grabbing, with Parker going down various instances during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Hatton Homage Fuels Manchester Derby Fight
In the secondary fight, Arthur beat Rea by divided decision in a terrific local rivalry fight to capture the European light heavyweight championship.
The venue became quiet for a deeply poignant memorial prior to the bout as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised fight night in the city since the passing of Hatton. As the concluding sound disappeared, the stillness became a chant of "there's just one Hatton".
The action inside the ropes equaled the emotion. The eventual winner was knocked down by a powerful punch squarely on the jaw in the early round, rising shakily to his position on the referee's count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured soon, noticeable damage developing under his face as the duo traded hard punches.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over the full distance.
The victor narrowly won the decision with scoring tallies of even score, narrow lead and 115-112, but each competitor were relentless and brave, all the qualities their inspiration the legend embodied.
