British light heavyweight Buatsi secured a victory yet underwhelming start to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round bout at the boxing arena.
But Parker found success, delivering effective clean blows in a contest where the approaches never quite clicked, with constant grabbing disrupting the fight.
Two judges judged it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They succeed," Buatsi commented in the ringside as he was greeted with disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts felt Parker, thirty-one, deserved the win but the Derbyshire fighter experienced the second loss of his 28-bout career.
The Champion Earns Victory in Slow Encounter
His career development has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of the British fight scene, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter ready to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who utilized a solid defense to narrowly win the better periods.
The audience's tolerance wore thin during frequent holding, with the challenger going down various instances during clinches.
Parker closed well, but many rounds were challenging to assess. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a excellent Manchester vs Manchester battle to secure the European light heavyweight championship.
The stadium became quiet for a profoundly moving memorial prior to the bout as a bell ceremony tolled in memory of the city's native Hatton, who died aged 46 in September.
It was the first major televised boxing event in the location since Hatton's death. As the last bell ended, the quiet became a roar of "there's just one Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a left hook directly to the face in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was marked up quickly, significant bruising appearing under his eye as the pair landed hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur barely secured the decision with judges scorecards of even score, close margin and wider gap, but the two athletes were persistent and brave, everything their idol the legend stood for.
