UK light-heavyweight Buatsi earned a win yet disappointing start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with frequent clinching marring the action.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what do champions do? They prevail," the victor said in the ringside as he was greeted with boos from the audience.
The Londoner got back to winning after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Various analysts believed the challenger, 31, deserved the win but the Derbyshire fighter suffered the second setback of his 28-match career.
The Champion Earns Victory in Uneventful Affair
The fighter's advancement has plateaued in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with both competitors willing to be aggressive in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was participating in his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who worked behind a tight guard to edge the more effective periods.
The audience's tolerance wore thin amid regular grabbing, with the challenger going down multiple occasions during clinches.
The underdog closed well, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Memorial Fuels All-Manchester Battle
In the co-main event, Arthur beat Rea by split-decision in a terrific all-Manchester fight to secure the European light heavyweight championship.
The venue fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony rang in honor of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The competition within the ring equaled the emotion. The eventual winner was sent to the canvas by a significant blow flush on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged quickly, significant bruising forming under his face as the fighters landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur barely secured the verdict with scoring tallies of draw, 115-113 and 115-112, but the two athletes were determined and courageous, everything their inspiration the legend stood for.
