British light-heavyweight Buatsi made a winning yet disappointing debut to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a competitive ten-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with constant grabbing spoiling the contest.
The officials scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest performance but what defines champions? They win," the victor commented in the arena as he was met by negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts thought Parker, thirty-one, merited the win but the contender experienced the second setback of his 28-fight career.
Buatsi Claims Win in Slow Match
His career development has slowed in recent times through contract problems and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now stands trying to achieve expectations - possibly in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to narrowly win the more effective periods.
The crowd's patience decreased because of regular grabbing, with Parker slipping to the canvas several times during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels Local Rivalry Battle
In the secondary fight, Arthur beat Rea by close verdict in a terrific Manchester vs Manchester battle to secure the continental belt.
The stadium grew hushed for a extremely emotional homage ahead of the contest as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell faded, the silence gave way to a shout of "only one Hatton".
The competition inside the ropes equaled the emotion. The eventual winner was sent to the canvas by a left hook flush on the chin in the second round, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was marked up soon, noticeable damage developing under his orbital area as the fighters exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur barely secured the decision with official scores of even score, narrow lead and wider gap, but each competitor were determined and courageous, all the qualities their idol Hatton embodied.
