UK light heavyweight Buatsi secured a victory yet disappointing debut to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round bout at the Manchester venue.
But Parker had his moments, delivering effective individual punches in a contest where the techniques never quite clicked, with constant grabbing spoiling the fight.
Two judges had it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest showing but what defines champions? They succeed," the victor commented in the ring as he was met by negative reactions from the spectators.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts believed the challenger, thirty-one, should have been awarded the win but the contender endured the second setback of his 28-bout professional journey.
The Champion Claims Triumph in Dull Encounter
His career development has slowed in the past few years through management issues and considerable time of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter 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 appearance at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who operated with a solid defense to barely take the better stanzas.
The crowd's patience diminished amid constant clinching, with Parker falling to the floor various instances during grappling.
Parker ended stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Russian champion Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Hatton Tribute Motivates Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding Manchester vs Manchester battle to win the continental title.
The stadium grew hushed for a deeply poignant homage before the fight as ten bells sounded in memory of the city's native Hatton, who passed away 46 years old in September.
It was the premier televised match card in the location since Hatton's passing. As the last bell disappeared, the stillness gave way to a shout of "only one Ricky Hatton".
The fighting during the bout equaled the feeling. The eventual winner was knocked down by a powerful punch flush on the jaw in the initial stage, standing uncertainly to his position on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. Across the ring, the opponent - in competitive blue - was damaged quickly, noticeable damage appearing under his face as the fighters landed punishing blows.
The challenger, 27, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the pace rarely dropped over the full distance.
Arthur edged the result with judges scorecards of draw, narrow lead and clearer margin, but both fighters were determined and full of heart, everything their inspiration the legend represented.
