British light-heavyweight Buatsi secured a victory yet underwhelming debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a tight ten-round bout at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with frequent clinching spoiling the fight.
Two judges had it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my finest performance but how do champions respond? They prevail," Buatsi said in the ringside as he was met by negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers believed Parker, 31, merited the victory but the Derbyshire fighter endured the second loss of his 28-match career.
The Champion Earns Victory in Slow Match
The fighter's advancement has slowed in recent years through contract problems and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to achieve expectations - perhaps in what feels like a last chance - under recent support.
He began cautiously, with the two boxers ready to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second appearance at light-heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who utilized a solid defense to barely take the more effective periods.
The crowd's patience wore thin amid constant clinching, with the challenger falling to the floor several times during grappling.
The underdog closed well, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester battle to secure the European light-heavyweight belt.
The venue grew hushed for a profoundly moving homage prior to the bout as a bell ceremony rang in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the location since Hatton's death. As the last bell ended, the silence gave way to a shout of "there's just one Hatton".
The fighting during the bout matched the emotion. The eventual winner was knocked down by a left hook flush on the chin in the second round, standing uncertainly to his feet on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was damaged soon, significant bruising appearing under his orbital area as the fighters traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the action hardly diminished over the full distance.
Arthur edged the decision with official scores of draw, close margin and clearer margin, but each competitor were relentless and brave, exactly what their inspiration Hatton embodied.
