UK light heavyweight Buatsi made a winning yet underwhelming debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close 10-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful individual punches in a fight where the approaches failed to mesh, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest showing but how do champions respond? They succeed," Buatsi said in the arena as he was greeted with disapproval from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Several boxing experts felt Parker, thirty-one, merited the decision but the contender suffered the second defeat of his 28-match professional journey.
Buatsi Claims Victory in Uneventful Affair
Buatsi's progress has slowed in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to achieve expectations - possibly in what seems like a final roll of the dice - under recent support.
He started carefully, with both competitors willing to overcommit in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after damaging his hand in the contest, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who operated with a protective stance to narrowly win the more effective rounds.
The spectators' acceptance diminished during frequent holding, with the challenger going down several times during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a outstanding local rivalry battle to win the continental championship.
The stadium fell silent for a extremely emotional memorial prior to the bout as 10 bells tolled in honor of Manchester's own Hatton, who died 46 years old in last month.
It was the first major televised match card in the city since Hatton's passing. As the last bell disappeared, the stillness became a shout of "there's just one Hatton".
The fighting inside the ropes equaled the sentiment. The eventual winner was floored by a powerful punch flush on the face in the initial stage, rising shakily to his footing on the count of eight.
But the red-colored equipment of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was damaged quickly, heavy swelling appearing under his orbital area as the duo exchanged powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur barely secured the decision with judges scorecards of draw, close margin and clearer margin, but both fighters were relentless and full of heart, all the qualities their inspiration the legend represented.
