UK light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive ten-round fight at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a fight where the styles didn't complement each other, with excessive holding disrupting the action.
The officials had it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best showing but how do champions respond? They win," Buatsi stated in the ring as he was greeted with boos from the audience.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Various analysts thought the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Earns Triumph in Uneventful Match
The fighter's advancement has slowed in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors willing to overcommit in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to edge the more effective rounds.
The audience's tolerance decreased because of regular grabbing, with the challenger slipping to the canvas various instances during clinches.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Homage Motivates Local Rivalry Battle
In the co-main event, Lyndon Arthur beat Rea by divided decision in a outstanding all-Manchester fight to secure the European light heavyweight belt.
The venue became quiet for a deeply poignant tribute ahead of the contest as 10 bells sounded in memory of Manchester's own Ricky Hatton, who died at age 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the final chime faded, the silence gave way to a shout of "there's only one Ricky Hatton".
The competition during the bout equaled the emotion. Arthur was sent to the canvas by a left hook directly to the jaw in the second round, getting up unsteadily to his position on the eight count.
But the red-colored shorts of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged quickly, noticeable damage appearing under his eye as the duo landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace seldom decreased over 12 rounds.
Arthur barely secured the decision with judges scorecards of draw, 115-113 and 115-112, but each competitor were persistent and full of heart, all the qualities their idol the legend represented.
