UK light heavyweight Buatsi made a winning yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker found success, connecting with powerful clean blows in a contest where the techniques didn't complement each other, with excessive holding spoiling the contest.
The officials had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best display but how do champions respond? They succeed," Buatsi stated in the ringside as he was met by disapproval from the audience.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers felt the challenger, 31, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
The Champion Earns Victory in Dull Match
Buatsi's progress has stalled in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the future of UK boxing, he now finds himself trying to achieve expectations - maybe in what feels like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter ready to overcommit in a cagey opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was participating in his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who operated with a tight guard to barely take the better periods.
The crowd's patience diminished amid frequent holding, with Parker falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but several stanzas were hard to judge. However, former world champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Inspires All-Manchester Contest
In the secondary fight, Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester fight to capture the continental belt.
The stadium became quiet for a extremely emotional memorial before the fight as 10 bells sounded in honor of the city's native Hatton, who died at age 46 in recently.
It was the first major televised match card in the city since Hatton's passing. As the concluding sound ended, the quiet gave way to a roar of "only one Ricky Hatton".
The action inside the ropes reflected the emotion. The eventual winner was floored by a powerful punch flush on the jaw in the early round, standing uncertainly to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was marked up soon, noticeable damage forming under his eye as the fighters traded hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the decision with judges scorecards of 114-114, close margin and 115-112, but both fighters were relentless and courageous, exactly what their idol the legend stood for.
