UK light-heavyweight Buatsi earned a win yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a tight ten-round contest at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a fight where the styles never quite clicked, with frequent clinching disrupting the action.
The officials scored it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They succeed," Buatsi commented in the ring as he was greeted with boos from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers felt Parker, thirty-one, merited the victory but the contender suffered the second defeat of his 28-fight professional journey.
Buatsi Claims Win in Dull Encounter
Buatsi's progress has slowed in recent years through management issues 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 - possibly in what appears to be a last chance - under new promotional backing.
He began cautiously, with neither fighter ready to take risks in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who utilized a tight guard to narrowly win the better stanzas.
The crowd's patience wore thin amid constant clinching, with Parker falling to the floor multiple occasions during clinches.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester contest to capture the European light-heavyweight championship.
The arena became quiet for a extremely emotional homage ahead of the contest as 10 bells rang in memory of the city's native Ricky Hatton, who died 46 years old in last month.
It was the first major televised boxing event in the city since the passing of Hatton. As the last bell faded, the quiet became a roar of "only one Ricky Hatton".
The action inside the ropes matched the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the face in the initial stage, rising shakily to his footing on the referee's count.
But the traditional red equipment of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was marked up early, noticeable damage developing under his face as the duo traded punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the verdict with judges scorecards of draw, narrow lead and wider gap, but both fighters were relentless and brave, everything their idol Hatton embodied.
