British light heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a close ten-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective single shots in a match where the techniques failed to mesh, with constant grabbing disrupting the contest.
The scoring judges had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what do champions do? They prevail," the victor said in the arena as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to secure a 20th pro success.
Various analysts felt the challenger, 31, deserved the win but the British boxer suffered the second setback of his 28-fight professional journey.
The Champion Claims Victory in Slow Affair
Buatsi's progress has slowed in recent years through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to realize his potential - maybe in what feels like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter willing to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second bout at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who utilized a protective stance to narrowly win the cleaner stanzas.
The crowd's patience diminished because of constant clinching, with the challenger slipping to the canvas various instances during clinches.
The underdog closed well, but many rounds were hard to judge. However, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi advances, but with the presence of power puncher Benavidez, and Russian champion Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Hatton Homage Motivates All-Manchester Battle
In the co-main event, Arthur defeated Bradley Rea by divided decision in a terrific all-Manchester fight to secure the European light-heavyweight championship.
The stadium fell silent for a profoundly moving homage prior to the bout as ten bells sounded in memory of local hero Hatton, who died at age 46 in recently.
It was the premier televised match card in the urban center since Hatton's passing. As the concluding sound faded, the silence became a roar of "there's just one Ricky Hatton".
The competition within the ring equaled the emotion. Arthur was sent to the canvas by a powerful punch flush on the chin in the second round, standing uncertainly to his footing on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was injured soon, significant bruising appearing under his orbital area as the fighters traded powerful shots.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over the full distance.
Arthur narrowly won the verdict with official scores of draw, close margin and 115-112, but both fighters were determined and brave, everything their inspiration Hatton represented.
