British light-heavyweight Buatsi made a winning yet unimpressive beginning to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive ten-round bout at the Manchester venue.
But Parker had his moments, landing some telling single shots in a match where the styles never quite clicked, with frequent clinching disrupting the contest.
The scoring judges scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best performance but what do champions do? They prevail," the victor said in the ringside as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Various analysts thought Parker, 31, deserved the win but the British boxer suffered the second loss of his 28-match professional journey.
Buatsi Secures Victory in Dull Encounter
Buatsi's progress has slowed in recent years through management issues and considerable time of lack of fights.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter willing to be aggressive in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who worked behind a tight guard to edge the better rounds.
The crowd's patience decreased amid constant clinching, with the challenger going down various instances during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to secure the continental championship.
The arena grew hushed for a profoundly moving homage ahead of the contest as 10 bells tolled in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the premier televised fight night in the city since Hatton's death. As the concluding sound ended, the quiet gave way to a chant of "there's only one Ricky Hatton".
The fighting during the bout reflected the feeling. The eventual winner was sent to the canvas by a significant blow flush on the chin in the second round, standing uncertainly to his position on the count of eight.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was injured soon, significant bruising forming under his eye as the duo landed hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over the full distance.
Arthur edged the verdict with official scores of 114-114, narrow lead and 115-112, but both fighters were determined and courageous, all the qualities their hero the legend represented.
