British light heavyweight Buatsi secured a victory yet disappointing debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round contest at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a match where the approaches never quite clicked, with constant grabbing marring the fight.
The officials judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my finest performance but what defines champions? They win," Buatsi commented in the ringside as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought Parker, thirty-one, deserved the win but the contender endured the second loss of his 28-fight career.
The Champion Earns Triumph in Dull Encounter
The fighter's advancement has slowed in the past few years through promotional disputes and extended periods of lack of fights.
Previously predicted as the next star of UK boxing, he now finds himself trying to realize his potential - possibly in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers ready to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was making just his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who utilized a tight guard to barely take the more effective periods.
The spectators' acceptance wore thin during frequent holding, with the challenger falling to the floor various instances during clinches.
Parker closed well, but multiple periods were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Homage Inspires Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester battle to win the continental title.
The arena fell silent for a extremely emotional tribute ahead of the contest as ten bells tolled in memory of Manchester's own Hatton, who succumbed at age 46 in September.
It was the first major televised fight night in the city since the passing of Hatton. As the concluding sound ended, the stillness transitioned to a chant of "there's just one Hatton".
The competition within the ring equaled the emotion. Arthur was knocked down by a powerful punch squarely on the chin in the second round, standing uncertainly to his position on the referee's count.
But the red-colored equipment of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was marked up soon, significant bruising forming under his orbital area as the duo exchanged hard punches.
Rea, 27, landed power punches but Arthur, 34, matched the intensity with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the result with official scores of 114-114, close margin and 115-112, but both fighters were relentless and courageous, everything their idol Hatton embodied.
