UK light heavyweight Buatsi secured a victory yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful individual punches in a fight where the approaches failed to mesh, with frequent clinching marring the fight.
The officials judged it 96-94 to the winner, with the third a even score.
"It wasn't my finest display but how do champions respond? They win," Buatsi commented in the ringside as he was met by negative reactions from the spectators.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts believed Parker, 31, should have been awarded the decision but the contender experienced the second setback of his 28-fight professional journey.
Buatsi Secures Win in Uneventful Match
His career development has plateaued in recent years through management issues and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors prepared to be aggressive in a tentative opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who operated with a protective stance to narrowly win the cleaner rounds.
The audience's tolerance wore thin because of constant clinching, with Parker going down several times during clinches.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Tribute Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a excellent all-Manchester battle to win the continental belt.
The stadium fell silent for a extremely emotional memorial ahead of the contest as 10 bells sounded in memory of the city's native Hatton, who succumbed at age 46 in recently.
It was the first major televised match card in the urban center since the passing of Hatton. As the concluding sound disappeared, the stillness gave way to a chant of "only one Hatton".
The action inside the ropes equaled the sentiment. The eventual winner was floored by a significant blow squarely on the chin in the early round, standing uncertainly to his position on the count of eight.
But the traditional red attire of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was marked up soon, heavy swelling appearing under his eye as the fighters landed hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over the full distance.
Arthur narrowly won the verdict with scoring tallies of even score, 115-113 and clearer margin, but both fighters were determined and full of heart, exactly what their hero Hatton represented.
