UK light-heavyweight Buatsi earned a win yet unimpressive debut to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a tight 10-round fight at the Manchester venue.
But Parker found success, landing some telling single shots in a fight where the styles never quite clicked, with constant grabbing disrupting the action.
Two judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest performance but what defines champions? They prevail," the victor commented in the ringside as he was greeted with disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Various analysts thought Parker, 31, should have been awarded the victory but the contender endured the second setback of his 28-bout career.
Buatsi Earns Triumph in Uneventful Match
The fighter's advancement has plateaued in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to achieve expectations - maybe in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a tentative opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was making just his second bout at light heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who utilized a tight guard to barely take the better stanzas.
The crowd's patience decreased amid regular grabbing, with Parker falling to the floor several times during close exchanges.
The underdog finished the stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a excellent all-Manchester contest to secure the European light-heavyweight championship.
The stadium became quiet for a deeply poignant homage ahead of the contest as 10 bells tolled in remembrance of the city's native Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast match card in the city since Hatton's passing. As the final chime disappeared, the stillness became a shout of "there's only one Hatton".
The action inside the ropes matched the feeling. Arthur was floored by a significant blow flush on the face in the early round, getting up unsteadily to his footing on the eight count.
But the red-colored attire of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was damaged quickly, heavy swelling developing under his eye as the pair traded hard punches.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action hardly diminished over the full distance.
Arthur edged the decision with judges scorecards of draw, 115-113 and 115-112, but each competitor were relentless and courageous, everything their inspiration the legend embodied.
