UK light-heavyweight Buatsi made a winning yet unimpressive start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the approaches never quite clicked, with excessive holding marring the contest.
The officials had it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They prevail," the victor said in the ring as he was faced boos from the audience.
The Londoner bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers believed Parker, 31, should have been awarded the win but the contender endured the second loss of his 28-match career.
The Champion Secures Win in Uneventful Match
Buatsi's progress has plateaued in recent times through promotional disputes and long spells of lack of fights.
Earlier forecast as the next star of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under recent support.
He began cautiously, with neither fighter prepared to be aggressive in a tentative initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was making just his second bout at light heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a solid defense to edge the better stanzas.
The spectators' acceptance wore thin amid constant clinching, with the challenger going down various instances during clinches.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Memorial Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a terrific local rivalry battle to capture the European light heavyweight belt.
The venue grew hushed for a extremely emotional homage ahead of the contest as 10 bells sounded in remembrance of Manchester's own Hatton, who passed away at age 46 in September.
It was the first major televised boxing event in the city since Hatton's passing. As the concluding sound ended, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The competition inside the ropes equaled the feeling. Arthur was sent to the canvas by a significant blow squarely on the face in the initial stage, standing uncertainly to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was injured early, heavy swelling developing under his orbital area as the pair exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur barely secured the decision with judges scorecards of even score, 115-113 and clearer margin, but each competitor were determined and courageous, everything their idol Hatton embodied.
