UK light-heavyweight Buatsi made a winning yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a tight 10-round bout at the Manchester venue.
But the challenger landed significant shots, connecting with powerful individual punches in a match where the approaches never quite clicked, with constant grabbing spoiling the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my greatest display but what defines champions? They succeed," the victor commented in the arena as he was greeted with disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers felt Parker, 31, merited the victory but the British boxer experienced the second defeat of his 28-bout professional journey.
Buatsi Secures Triumph in Slow Match
Buatsi's progress has stalled in recent times through promotional disputes and long spells of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to achieve expectations - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors ready to take risks in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury mid-fight, was making just his second bout at light heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who operated with a solid defense to narrowly win the cleaner rounds.
The crowd's patience wore thin amid frequent holding, with Parker slipping to the canvas multiple occasions during clinches.
The underdog closed well, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor advances, but with the presence of hard hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a outstanding local rivalry fight to secure the European light-heavyweight belt.
The stadium grew hushed for a extremely emotional memorial ahead of the contest as ten bells rang in memory of local hero Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the concluding sound faded, the silence became a roar of "there's only one Hatton".
The fighting inside the ropes reflected the feeling. Arthur was knocked down by a left hook directly to the jaw in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, the opponent - in competitive blue - was injured quickly, noticeable damage developing under his eye as the pair exchanged hard punches.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over the full distance.
Arthur barely secured the decision with judges scorecards of draw, narrow lead and wider gap, but each competitor were relentless and full of heart, exactly what their hero Hatton stood for.
