UK light heavyweight Buatsi made a winning yet underwhelming debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a contest where the styles never quite clicked, with excessive holding disrupting the fight.
Two judges had it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my best showing but what defines champions? They succeed," Buatsi commented in the ringside as he was greeted with negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Various analysts thought the challenger, thirty-one, deserved the win but the Derbyshire fighter suffered the second loss of his 28-bout career.
The Champion Claims Triumph in Slow Affair
His career development has stalled in recent times through contract problems and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under recent support.
He began cautiously, with neither fighter willing to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who worked behind a protective stance to edge the better periods.
The spectators' acceptance wore thin because of constant clinching, with Parker falling to the floor various instances during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester battle to capture the European light-heavyweight championship.
The arena fell silent for a extremely emotional tribute ahead of the contest as ten bells rang in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the first major televised match card in the urban center since the passing of Hatton. As the last bell disappeared, the silence transitioned to a chant of "only one Hatton".
The fighting inside the ropes reflected the emotion. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his position on the referee's count.
But the red-colored attire of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was marked up early, significant bruising forming under his orbital area as the fighters landed punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the action rarely dropped over 12 rounds.
Arthur edged the verdict with official scores of draw, close margin and wider gap, but both fighters were persistent and courageous, exactly what their inspiration Hatton stood for.
