UK light heavyweight Buatsi made a winning yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a contest where the techniques didn't complement each other, with frequent clinching disrupting the fight.
The scoring judges scored it 96-94 for to the winner, with the third a even score.
"It wasn't my best performance but what defines champions? They prevail," Buatsi stated in the ring as he was greeted with disapproval from the crowd.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Several boxing experts thought the challenger, 31, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-match career.
The Champion Secures Victory in Dull Affair
The fighter's advancement has stalled in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He began cautiously, with both competitors willing to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who worked behind a solid defense to narrowly win the more effective periods.
The spectators' acceptance wore thin amid frequent holding, with the challenger falling to the floor various instances during clinches.
The underdog closed well, but several stanzas were hard to judge. Although, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Arthur defeated Bradley Rea by divided decision in a outstanding local rivalry contest to capture the European light-heavyweight championship.
The arena fell silent for a profoundly moving tribute before the fight as 10 bells rang in honor of local hero Hatton, who passed away at age 46 in recently.
It was the first major televised match card in the urban center since Hatton's death. As the last bell disappeared, the stillness transitioned to a shout of "there's only one Hatton".
The fighting during the bout matched the emotion. The eventual winner was knocked down by a left hook directly to the chin in the early round, standing uncertainly to his position on the referee's count.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured quickly, heavy swelling forming under his eye as the fighters landed hard punches.
Rea, 27, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur edged the decision with judges scorecards of draw, close margin and 115-112, but both fighters were determined and full of heart, exactly what their hero the legend embodied.
