UK light-heavyweight Buatsi earned a win yet disappointing start to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a match where the approaches didn't complement each other, with frequent clinching marring the fight.
The officials had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best display but what do champions do? They prevail," Buatsi stated in the ringside as he was faced disapproval from the crowd.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Various analysts felt the challenger, 31, should have been awarded the win but the British boxer experienced the second loss of his 28-match career.
The Champion Earns Triumph in Uneventful Encounter
The fighter's advancement has stalled in recent years through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second fight at light heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to narrowly win the more effective stanzas.
The audience's tolerance decreased during constant clinching, with the challenger going down various instances during clinches.
Parker finished the stronger, but many rounds were difficult to score. Although, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Inspires All-Manchester Contest
In the secondary fight, Arthur beat Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to capture the European light heavyweight belt.
The venue grew hushed for a profoundly moving memorial prior to the bout as 10 bells sounded in honor of Manchester's own Hatton, who passed away at age 46 in last month.
It was the first major televised match card in the location since Hatton's passing. As the last bell disappeared, the silence gave way to a shout of "only one Hatton".
The competition within the ring equaled the emotion. Arthur was sent to the canvas by a left hook squarely on the face in the second round, getting up unsteadily to his feet on the referee's count.
But the traditional red equipment of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was damaged soon, significant bruising developing under his face as the fighters traded hard punches.
Rea, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over twelve rounds.
The victor edged the decision with official scores of 114-114, 115-113 and clearer margin, but both fighters were determined and courageous, exactly what their idol Hatton embodied.
