British light heavyweight Buatsi secured a victory yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive ten-round bout at the Manchester venue.
But the challenger landed significant shots, connecting with powerful individual punches in a match where the techniques never quite clicked, with excessive holding spoiling the contest.
Two judges scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They prevail," the victor said in the arena as he was greeted with negative reactions from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Various analysts thought the challenger, 31, deserved the victory but the Derbyshire fighter endured the second setback of his 28-bout career.
The Champion Claims Triumph in Uneventful Affair
Buatsi's progress has slowed in recent years through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with both competitors prepared to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was competing in only his second fight at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to barely take the more effective periods.
The audience's tolerance decreased because of constant clinching, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding 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 seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry contest to secure the European light-heavyweight title.
The venue grew hushed for a extremely emotional tribute ahead of the contest as 10 bells sounded in honor of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the concluding sound disappeared, the silence transitioned to a chant of "only one Hatton".
The competition within the ring reflected the sentiment. Arthur was knocked down by a left hook flush on the chin in the second round, getting up unsteadily to his position on the referee's count.
But the red-colored attire of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was injured quickly, heavy swelling forming under his eye as the pair landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur narrowly won the verdict with official scores of draw, 115-113 and wider gap, but both fighters were persistent and full of heart, everything their hero Hatton represented.
