British light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the techniques didn't complement each other, with frequent clinching spoiling the action.
The officials scored it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my finest performance but what defines champions? They win," the victor stated in the ring as he was met by negative reactions from the audience.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Various analysts believed the challenger, thirty-one, deserved the decision but the Derbyshire fighter experienced the second defeat of his 28-fight career.
The Champion Claims Victory in Slow Encounter
His career development has stalled in recent years through promotional disputes and extended periods of lack of fights.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with both competitors prepared to take risks in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who operated with a solid defense to narrowly win the more effective rounds.
The audience's tolerance diminished during frequent holding, with the challenger falling to the floor multiple occasions during grappling.
The underdog closed well, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Homage Inspires Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a outstanding local rivalry fight to capture the European light-heavyweight belt.
The stadium became quiet for a deeply poignant memorial prior to the bout as ten bells rang in remembrance of the city's native Hatton, who died 46 years old in recently.
It was the first major televised fight night in the urban center since Hatton's passing. As the concluding sound faded, the stillness gave way to a chant of "there's only one Ricky Hatton".
The action during the bout reflected the feeling. The eventual winner was knocked down by a left hook directly to the face in the second round, rising shakily to his position on the eight count.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was marked up soon, heavy swelling appearing under his eye as the pair traded hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the pace seldom decreased over 12 rounds.
Arthur narrowly won the result with official scores of draw, 115-113 and 115-112, but both fighters were determined and full of heart, everything their idol Hatton stood for.
