British light heavyweight Buatsi made a winning yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a competitive 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling individual punches in a contest where the styles didn't complement each other, with frequent clinching marring the fight.
Two judges scored it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my best showing but what do champions do? They succeed," the victor stated in the ringside as he was met by negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Some ringside observers felt Parker, 31, merited the victory but the British boxer suffered the second defeat of his 28-fight career.
Buatsi Claims Victory in Slow Affair
Buatsi's progress has plateaued in recent years through contract problems and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with neither fighter prepared to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second appearance at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to narrowly win the better stanzas.
The crowd's patience wore thin amid constant clinching, with the challenger going down several times during clinches.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor moves on, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry fight to secure the continental title.
The venue became quiet for a profoundly moving homage before the fight as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who succumbed at age 46 in last month.
It was the premier televised fight night in the city since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The action within the ring equaled the emotion. The eventual winner was knocked down by a powerful punch flush on the jaw in the initial stage, rising shakily to his position on the referee's count.
But the red-colored shorts of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was injured soon, significant bruising forming under his face as the pair traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur edged the result with scoring tallies of 114-114, narrow lead and 115-112, but both fighters were determined and courageous, everything their inspiration Hatton represented.
