UK light-heavyweight Buatsi earned a win yet disappointing debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round contest at the Manchester venue.
But Parker had his moments, landing some telling single shots in a match where the techniques didn't complement each other, with excessive holding disrupting the fight.
The officials judged it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi commented in the ringside as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts believed the challenger, 31, deserved the decision but the contender experienced the second defeat of his 28-fight career.
The Champion Secures Triumph in Dull Encounter
His career development has stalled in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with both competitors willing to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand mid-fight, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who worked behind a protective stance to barely take the better rounds.
The spectators' acceptance decreased during frequent holding, with the challenger slipping to the canvas various instances during grappling.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Memorial Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a outstanding all-Manchester battle to win the continental title.
The stadium grew hushed for a extremely emotional tribute prior to the bout as ten bells sounded in honor of the city's native Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the urban center since Hatton's passing. As the final chime faded, the stillness became a roar of "there's just one Hatton".
The action during the bout equaled the feeling. The eventual winner was floored by a left hook directly to the chin in the initial stage, getting up unsteadily to his position on the referee's count.
But the red-colored attire of Arthur soon advanced again. Across the ring, the opponent - in competitive blue - was injured soon, significant bruising developing under his orbital area as the fighters landed hard punches.
Rea, 27, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the decision with official scores of 114-114, 115-113 and clearer margin, but both fighters were relentless and brave, exactly what their hero the legend represented.
