UK light-heavyweight Buatsi secured a victory yet unimpressive start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a split decision in a competitive ten-round contest at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the styles never quite clicked, with frequent clinching marring the contest.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but what defines champions? They win," the victor said in the ringside as he was met by disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Various analysts believed the challenger, thirty-one, should have been awarded the victory but the British boxer endured the second setback of his 28-fight career.
Buatsi Claims Win in Dull Encounter
His career development has stalled in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the future of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a last chance - under new promotional backing.
He began cautiously, with both competitors ready to overcommit in a calculated opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who operated with a tight guard to narrowly win the better stanzas.
The audience's tolerance decreased because of constant clinching, with Parker falling to the floor several times during grappling.
Parker finished the stronger, but multiple periods were difficult to score. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the continental championship.
The arena became quiet for a profoundly moving memorial ahead of the contest as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised boxing event in the urban center since Hatton's death. As the final chime faded, the stillness became a roar of "there's only one Hatton".
The action inside the ropes matched the feeling. Arthur was knocked down by a left hook flush on the face in the second round, rising shakily to his feet on the referee's count.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was marked up early, significant bruising forming under his face as the fighters traded hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor edged the verdict with official scores of 114-114, close margin and 115-112, but each competitor were determined and full of heart, everything their hero the legend represented.
