British light heavyweight Buatsi secured a victory yet underwhelming start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But Parker found success, landing some telling clean blows in a match where the approaches failed to mesh, with frequent clinching spoiling the action.
Two judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but how do champions respond? They succeed," Buatsi stated in the ringside as he was greeted with boos from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts thought the challenger, thirty-one, merited the win but the British boxer experienced the second defeat of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Match
Buatsi's progress has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what appears to be a last chance - under fresh management.
He opened tentatively, with the two boxers willing to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second fight at light heavyweight.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who worked behind a solid defense to narrowly win the cleaner stanzas.
The crowd's patience wore thin because of frequent holding, with Parker going down several times during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Arthur overcame Rea by split-decision in a excellent Manchester vs Manchester battle to win the European light-heavyweight title.
The stadium grew hushed for a profoundly moving homage before the fight as ten bells rang in honor of local hero Hatton, who died 46 years old in recently.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the last bell faded, the quiet became a roar of "there's just one Hatton".
The competition inside the ropes reflected the sentiment. Arthur was knocked down by a left hook flush on the jaw in the initial stage, standing uncertainly to his feet on the count of eight.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was marked up quickly, heavy swelling developing under his eye as the duo traded powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur edged the result with official scores of even score, close margin and 115-112, but both fighters were determined and brave, all the qualities their hero Hatton embodied.
