UK light heavyweight Buatsi secured a victory yet underwhelming debut to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight ten-round fight at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a match where the styles didn't complement each other, with frequent clinching disrupting the action.
The officials scored it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They win," Buatsi said in the ring as he was greeted with disapproval from the crowd.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts thought Parker, thirty-one, deserved the win but the British boxer endured the second setback of his 28-match career.
Buatsi Claims Victory in Uneventful Affair
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the future of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under new promotional backing.
He started carefully, with neither fighter prepared to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to edge the cleaner stanzas.
The crowd's patience decreased because of regular grabbing, with Parker falling to the floor various instances during grappling.
Parker closed well, but many rounds were difficult to score. However, ex-champion Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a terrific all-Manchester fight to secure the continental championship.
The venue grew hushed for a deeply poignant homage ahead of the contest as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell ended, the quiet gave way to a chant of "only one Hatton".
The fighting within the ring reflected the sentiment. Arthur was sent to the canvas by a significant blow directly to the jaw in the early round, standing uncertainly to his position on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was injured quickly, significant bruising forming under his eye as the pair landed hard punches.
Rea, 27, landed power punches but Arthur, 34, matched the intensity with stinging uppercuts as the pace seldom decreased over 12 rounds.
Arthur barely secured the decision with official scores of even score, narrow lead and clearer margin, but each competitor were relentless and brave, all the qualities their inspiration Hatton embodied.
