UK light-heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close 10-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a match where the styles failed to mesh, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They prevail," Buatsi commented in the ringside as he was faced disapproval from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Various analysts felt Parker, 31, deserved the decision but the Derbyshire fighter suffered the second defeat of his 28-fight career.
Buatsi Claims Triumph in Slow Affair
His career development has stalled in recent times through promotional disputes and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers willing to take risks in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was making just his second fight at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who operated with a protective stance to narrowly win the better rounds.
The audience's tolerance diminished because of constant clinching, with the challenger going down various instances during close exchanges.
The underdog closed well, but several stanzas were hard to judge. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Motivates Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent local rivalry contest to capture the European light heavyweight belt.
The arena fell silent for a deeply poignant memorial before the fight as 10 bells tolled in remembrance of Manchester's own Hatton, who died 46 years old in last month.
It was the initial significant broadcast match card in the location since Hatton's death. As the last bell disappeared, the stillness transitioned to a roar of "there's only one Ricky Hatton".
The competition during the bout reflected the sentiment. Arthur was knocked down by a powerful punch squarely on the jaw in the early round, rising shakily to his feet on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in competitive blue - was injured quickly, heavy swelling forming under his eye as the fighters landed powerful shots.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor barely secured the verdict with scoring tallies of draw, 115-113 and wider gap, but the two athletes were relentless and full of heart, exactly what their inspiration the legend stood for.
