UK light-heavyweight Buatsi earned a win yet unimpressive start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling single shots in a contest where the approaches didn't complement each other, with frequent clinching disrupting the contest.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my greatest display but what defines champions? They win," Buatsi said in the ringside as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers thought Parker, 31, should have been awarded the victory but the Derbyshire fighter endured the second defeat of his 28-match career.
The Champion Claims Victory in Dull Match
His career development has slowed in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a last chance - under fresh management.
He started carefully, with neither fighter willing to take risks in a cagey initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who worked behind a tight guard to barely take the cleaner stanzas.
The crowd's patience decreased because of regular grabbing, with Parker going down multiple occasions during clinches.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor 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 road to world glory - based on this performance - still appears challenging.
Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by split-decision in a terrific Manchester vs Manchester contest to capture the continental championship.
The venue fell silent for a profoundly moving homage prior to the bout as ten bells rang in honor of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the first major televised fight night in the urban center since Hatton's passing. As the final chime disappeared, the quiet transitioned to a roar of "there's just one Hatton".
The competition during the bout matched the feeling. Arthur was floored by a left hook squarely on the face in the second round, standing uncertainly to his footing on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured soon, heavy swelling appearing under his eye as the pair traded hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur edged the result with scoring tallies of even score, 115-113 and 115-112, but the two athletes were determined and brave, everything their inspiration Hatton embodied.
