UK light heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight 10-round contest at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the styles never quite clicked, with frequent clinching disrupting the contest.
Two judges judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest showing but how do champions respond? They succeed," Buatsi stated in the arena as he was met by negative reactions from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts felt the challenger, 31, deserved the win but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
Buatsi Secures Victory in Dull Match
Buatsi's progress has stalled in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter prepared to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who utilized a tight guard to edge the more effective stanzas.
The audience's tolerance diminished because of constant clinching, with Parker going down various instances during close exchanges.
Parker closed well, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by divided decision in a terrific local rivalry fight to capture the continental title.
The venue grew hushed for a extremely emotional homage before the fight as 10 bells tolled in memory of the city's native Ricky Hatton, who succumbed 46 years old in September.
It was the premier televised match card in the urban center since Hatton's death. As the final chime faded, the silence transitioned to a chant of "only one Hatton".
The action within the ring equaled the emotion. The eventual winner was floored by a significant blow flush on the chin in the second round, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was marked up early, noticeable damage developing under his face as the duo landed punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over the full distance.
The victor narrowly won the verdict with official scores of even score, narrow lead and clearer margin, but both fighters were determined and brave, all the qualities their idol the legend represented.
