British light-heavyweight Buatsi made a winning yet disappointing debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a split decision in a tight ten-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling individual punches in a fight where the styles didn't complement each other, with excessive holding spoiling the action.
The scoring judges had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest performance but how do champions respond? They prevail," Buatsi commented in the arena as he was greeted with disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Various analysts thought Parker, 31, should have been awarded the win but the Derbyshire fighter endured the second defeat of his 28-bout career.
The Champion Earns Triumph in Dull Encounter
The fighter's advancement has plateaued in recent years through contract problems and long spells of lack of fights.
Once tipped as the next star of UK boxing, he now is positioned trying to realize his potential - possibly in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter willing to be aggressive in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who operated with a tight guard to edge the better rounds.
The crowd's patience wore thin amid frequent holding, with Parker slipping to the canvas various instances during clinches.
Parker finished the stronger, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Hatton Homage Motivates Manchester Derby Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific all-Manchester contest to capture the European light heavyweight championship.
The venue grew hushed for a extremely emotional homage prior to the bout as 10 bells sounded in remembrance of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the city since Hatton's passing. As the final chime faded, the quiet transitioned to a roar of "there's just one Hatton".
The action during the bout equaled the sentiment. The eventual winner was sent to the canvas by a left hook flush on the chin in the initial stage, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was damaged quickly, heavy swelling forming under his eye as the fighters traded powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over the full distance.
The victor edged the result with judges scorecards of draw, close margin and 115-112, but each competitor were persistent and courageous, everything their inspiration Hatton stood for.
