British light-heavyweight Buatsi secured a victory yet disappointing start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the styles never quite clicked, with excessive holding marring the action.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best performance but what defines champions? They succeed," Buatsi commented in the ring as he was met by negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th pro success.
Various analysts felt the challenger, thirty-one, deserved the victory but the British boxer suffered the second setback of his 28-bout career.
The Champion Secures Victory in Slow Affair
Buatsi's progress has stalled in the past few years through contract problems and considerable time of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a last chance - under fresh management.
He began cautiously, with both competitors prepared to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second bout 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 edge the better periods.
The audience's tolerance diminished during regular grabbing, with Parker falling to the floor several times during clinches.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Memorial Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a outstanding all-Manchester fight to capture the European light heavyweight championship.
The venue grew hushed for a extremely emotional homage ahead of the contest as a bell ceremony rang in remembrance of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the last bell ended, the stillness became a shout of "only one Hatton".
The action inside the ropes reflected the emotion. Arthur was floored by a left hook directly to the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the red-colored shorts of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was damaged quickly, significant bruising developing under his orbital area as the fighters exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor narrowly won the result with official scores of draw, narrow lead and clearer margin, but the two athletes were determined and full of heart, all the qualities their inspiration Hatton stood for.
