UK light heavyweight Buatsi made a winning yet underwhelming debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a tight 10-round contest at the boxing arena.
But Parker had his moments, connecting with powerful individual punches in a contest where the approaches didn't complement each other, with frequent clinching disrupting the action.
The officials scored it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best display but how do champions respond? They prevail," Buatsi said in the ring as he was met by boos from the crowd.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Various analysts thought Parker, 31, merited the decision but the contender suffered the second loss of his 28-match career.
Buatsi Claims Win in Uneventful Match
His career development has stalled in recent times through promotional disputes and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to realize his potential - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter willing to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who utilized a solid defense to barely take the more effective periods.
The crowd's patience diminished amid frequent holding, with Parker falling to the floor several times during close exchanges.
Parker finished the stronger, but many rounds were challenging to assess. However, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent local rivalry fight to capture the European light heavyweight championship.
The arena grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony rang in remembrance of the city's native Hatton, who passed away at age 46 in recently.
It was the first major televised fight night in the urban center since the passing of Hatton. As the final chime disappeared, the quiet became a chant of "there's only one Hatton".
The action during the bout reflected the emotion. The eventual winner was sent to the canvas by a significant blow squarely on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red attire of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was damaged early, noticeable damage appearing under his face as the fighters traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over the full distance.
The victor edged the result with official scores of draw, narrow lead and clearer margin, but the two athletes were determined and brave, all the qualities their hero Hatton embodied.
