UK light heavyweight Buatsi earned a win yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a close ten-round contest at the boxing arena.
But the challenger landed significant shots, connecting with powerful single shots in a contest where the techniques failed to mesh, with frequent clinching spoiling the contest.
The officials had it 96-94 to the winner, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They prevail," the victor said in the arena as he was faced disapproval from the audience.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts believed the challenger, 31, merited the decision but the Derbyshire fighter suffered the second loss of his 28-bout career.
The Champion Earns Triumph in Slow Affair
The fighter's advancement has plateaued in recent times through management issues and considerable time of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a last chance - under fresh management.
He opened tentatively, with both competitors willing to overcommit in a cagey opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to barely take the more effective stanzas.
The crowd's patience decreased during constant clinching, with the challenger going down various instances during clinches.
Parker closed well, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Inspires Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to secure the European light heavyweight belt.
The venue grew hushed for a profoundly moving tribute before the fight as a bell ceremony rang in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised match card in the urban center since Hatton's death. As the last bell disappeared, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The action within the ring matched the emotion. Arthur was sent to the canvas by a left hook flush on the jaw in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red attire of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured quickly, heavy swelling appearing under his eye as the duo traded powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over twelve rounds.
Arthur edged the result with official scores of 114-114, close margin and clearer margin, but the two athletes were relentless and brave, everything their inspiration the legend stood for.
