UK light-heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a competitive 10-round contest at the boxing arena.
But Parker found success, landing some telling single shots in a fight where the approaches didn't complement each other, with excessive holding disrupting the fight.
The scoring judges had it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They succeed," the victor commented in the arena as he was faced boos from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Various analysts felt the challenger, thirty-one, should have been awarded the decision but the British boxer experienced the second defeat of his 28-match career.
The Champion Claims Win in Uneventful Match
The fighter's advancement has stalled in recent years through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to realize his potential - perhaps in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter willing to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a solid defense to narrowly win the cleaner rounds.
The spectators' acceptance decreased amid constant clinching, with Parker falling to the floor various instances during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. Although, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the likes of power puncher Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Tribute Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific local rivalry battle to secure the continental title.
The stadium grew hushed for a extremely emotional homage prior to the bout as 10 bells rang in honor of the city's native Hatton, who died 46 years old in September.
It was the first major televised boxing event in the location since Hatton's death. As the last bell disappeared, the quiet became a chant 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 jaw in the second round, standing uncertainly to his position on the referee's count.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up early, heavy swelling forming under his face as the fighters landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the action rarely dropped over 12 rounds.
The victor narrowly won the result with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were relentless and full of heart, everything their idol Hatton represented.
