UK light heavyweight Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a close ten-round contest at the boxing arena.
But Parker found success, landing some telling individual punches in a match where the techniques never quite clicked, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest performance but what do champions do? They prevail," Buatsi commented in the ringside as he was greeted with boos from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts felt Parker, thirty-one, deserved the decision but the Derbyshire fighter endured the second loss of his 28-bout career.
The Champion Earns Victory in Uneventful Match
Buatsi's progress has slowed in recent years through contract problems and considerable time of lack of fights.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter ready to take risks in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who operated with a tight guard to narrowly win the more effective rounds.
The crowd's patience diminished because of regular grabbing, with the challenger slipping to the canvas several times during clinches.
Parker ended stronger, but multiple periods were hard to judge. However, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor moves on, but with the caliber of heavy hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Fuels Local Rivalry Contest
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a excellent local rivalry contest to secure the continental title.
The venue fell silent for a deeply poignant tribute before the fight as a bell ceremony rang in memory of local hero Ricky Hatton, who passed away at age 46 in September.
It was the initial significant broadcast fight night in the city since Hatton's death. As the last bell faded, the quiet became a chant of "only one Hatton".
The fighting within the ring matched the emotion. Arthur was sent to the canvas by a significant blow squarely on the jaw in the second round, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was marked up quickly, significant bruising appearing under his eye as the duo exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the result with scoring tallies of draw, close margin and clearer margin, but the two athletes were relentless and brave, all the qualities their idol Hatton stood for.
