British light-heavyweight Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a competitive ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the approaches failed to mesh, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest performance but what defines champions? They succeed," the victor stated in the arena as he was faced boos from the audience.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts thought the challenger, thirty-one, deserved the decision but the British boxer suffered the second setback of his 28-fight professional journey.
The Champion Earns Win in Dull Affair
The fighter's advancement has plateaued in recent times through contract problems and long spells of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a last chance - under recent support.
He began cautiously, with both competitors prepared to overcommit in a cagey early stages.
The challenger, whose single earlier setback 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 appearance at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to narrowly win the more effective rounds.
The crowd's patience wore thin during constant clinching, with the challenger slipping to the canvas several times during close exchanges.
Parker closed well, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a outstanding local rivalry fight to capture the continental belt.
The arena became quiet for a profoundly moving tribute prior to the bout as a bell ceremony rang in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the concluding sound faded, the silence gave way to a roar of "there's just one Ricky Hatton".
The action during the bout reflected the feeling. Arthur was floored by a powerful punch directly to the chin in the second round, rising shakily to his feet on the eight count.
But the traditional red equipment of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was marked up quickly, noticeable damage developing under his face as the duo traded hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with powerful upward punches as the action rarely dropped over twelve rounds.
Arthur barely secured the verdict with scoring tallies of even score, close margin and clearer margin, but both fighters were relentless and courageous, all the qualities their idol Hatton stood for.
