UK light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close ten-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful clean blows in a contest where the styles didn't complement each other, with frequent clinching spoiling the fight.
The scoring judges scored it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi commented in the ring as he was greeted with 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 February - to claim a twentieth professional win.
Various analysts thought Parker, thirty-one, merited the win but the British boxer experienced the second loss of his 28-fight career.
Buatsi Earns Victory in Dull Encounter
Buatsi's progress has slowed in the past few years through management issues and long spells of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers willing to overcommit in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who utilized a protective stance to narrowly win the cleaner periods.
The crowd's patience diminished during regular grabbing, with Parker going down various instances during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. Although, former world champion the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Local Rivalry Fight
In the co-main event, Arthur beat Bradley Rea by close verdict in a outstanding local rivalry contest to secure the European light-heavyweight championship.
The stadium grew hushed for a deeply poignant homage before the fight as 10 bells rang in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the first major televised fight night in the city since the passing of Hatton. As the final chime ended, the silence became a shout of "there's just one Hatton".
The competition during the bout reflected the emotion. Arthur was sent to the canvas by a powerful punch flush on the face in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up soon, noticeable damage appearing under his face as the duo landed punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over the full distance.
The victor barely secured the result with scoring tallies of even score, close margin and wider gap, but both fighters were relentless and courageous, everything their inspiration Hatton represented.
