British light-heavyweight Buatsi made a winning yet disappointing start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive ten-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful clean blows in a contest where the approaches failed to mesh, with excessive holding disrupting the fight.
The scoring judges had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi said in the ring as he was greeted with disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought Parker, 31, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-match career.
Buatsi Earns Win in Slow Affair
The fighter's advancement has stalled in recent years through management issues and long spells of lack of fights.
Earlier forecast as the next star of UK boxing, he now stands trying to realize his potential - perhaps in what seems like a last chance - under fresh management.
He started carefully, with neither fighter prepared to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who operated with a protective stance to narrowly win the more effective periods.
The spectators' acceptance decreased amid constant clinching, with Parker falling to the floor various instances during clinches.
Parker ended stronger, but many rounds were hard to judge. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Fuels All-Manchester Contest
In the supporting bout, Arthur defeated Rea by split-decision in a terrific all-Manchester contest to capture the European light-heavyweight championship.
The venue fell silent for a profoundly moving homage before the fight as a bell ceremony tolled in remembrance of the city's native Ricky Hatton, who passed away 46 years old in September.
It was the initial significant broadcast fight night in the location since Hatton's death. As the final chime disappeared, the silence gave way to a shout of "there's just one Ricky Hatton".
The fighting during the bout equaled the emotion. The eventual winner was floored by a significant blow directly to the chin in the second round, standing uncertainly to his footing on the eight count.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged soon, significant bruising developing under his eye as the pair landed hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the action rarely dropped over the full distance.
Arthur barely secured the verdict with judges scorecards of even score, close margin and wider gap, but the two athletes were relentless and brave, all the qualities their inspiration Hatton stood for.
