UK light heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive 10-round bout at the boxing arena.
But Parker found success, delivering effective individual punches in a fight where the approaches failed to mesh, with excessive holding spoiling the fight.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi commented in the ring as he was faced boos from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Various analysts thought the challenger, 31, should have been awarded the victory but the British boxer suffered the second loss of his 28-match career.
The Champion Secures Victory in Slow Encounter
His career development has slowed in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to be aggressive in a calculated initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to edge the better rounds.
The crowd's patience decreased because of frequent holding, with Parker falling to the floor several times during close exchanges.
The underdog ended stronger, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a terrific Manchester vs Manchester battle to capture the European light heavyweight belt.
The stadium fell silent for a deeply poignant homage ahead of the contest as 10 bells rang in memory of the city's native Ricky Hatton, who died at age 46 in last month.
It was the premier televised match card in the city since Hatton's passing. As the last bell ended, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The fighting within the ring reflected the sentiment. The eventual winner was sent to the canvas by a left hook flush on the chin in the second round, standing uncertainly to his position on the count of eight.
But the traditional red shorts of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was injured quickly, significant bruising forming under his orbital area as the duo exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
The victor edged the decision with judges scorecards of 114-114, close margin and wider gap, but the two athletes were persistent and full of heart, exactly what their hero Hatton embodied.
