UK light heavyweight Buatsi secured a victory yet unimpressive beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a competitive 10-round contest at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a fight where the approaches never quite clicked, with frequent clinching marring the fight.
The officials scored it 96-94 in favor to the winner, with the third a even score.
"It wasn't my greatest showing but what defines champions? They win," the victor stated in the arena as he was met by disapproval from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Various analysts felt the challenger, 31, merited the win but the British boxer endured the second defeat of his 28-fight professional journey.
The Champion Secures Win in Uneventful Match
Buatsi's progress has slowed in the past few years through contract problems and considerable time of lack of fights.
Once tipped as the future of UK boxing, he now stands trying to realize his potential - maybe in what appears to be a last chance - under fresh management.
He opened tentatively, with the two boxers prepared to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second appearance at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to narrowly win the better rounds.
The spectators' acceptance diminished amid constant clinching, with the challenger falling to the floor various instances during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor advances, but with the likes of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Fight
In the supporting bout, Arthur defeated Rea by close verdict in a outstanding all-Manchester contest to secure the continental belt.
The venue fell silent for a extremely emotional tribute before the fight as 10 bells sounded in honor of the city's native Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the last bell ended, the quiet transitioned to a shout of "only one Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was floored by a powerful punch directly to the face in the initial stage, getting up unsteadily to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was damaged quickly, heavy swelling appearing under his orbital area as the fighters landed punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur narrowly won the decision with official scores of draw, 115-113 and 115-112, but the two athletes were persistent and full of heart, everything their inspiration Hatton represented.
