British light heavyweight Buatsi made a winning yet underwhelming start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a close ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful individual punches in a contest where the techniques never quite clicked, with frequent clinching marring the contest.
Two judges had it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They succeed," the victor stated in the ring as he was met by negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Some ringside observers felt Parker, thirty-one, merited the decision but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
Buatsi Claims Victory in Slow Match
His career development has stalled in the past few years through contract problems and considerable time of lack of fights.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - maybe in what feels like a final roll of the dice - under fresh management.
He started carefully, with both competitors prepared to overcommit in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, creating difficulties for Buatsi, who utilized a protective stance to edge the more effective periods.
The spectators' acceptance wore thin during regular grabbing, with Parker going down several times during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. Although, former world champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester fight to win the continental belt.
The venue became quiet for a extremely emotional tribute prior to the bout as ten bells tolled in memory of Manchester's own Hatton, who died aged 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the last bell ended, the silence transitioned to a shout of "there's just one Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a left hook flush on the jaw in the early round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was marked up early, noticeable damage developing under his face as the duo traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the action rarely dropped over 12 rounds.
Arthur narrowly won the verdict with scoring tallies of draw, close margin and clearer margin, but each competitor were determined and courageous, exactly what their idol the legend represented.
