British light heavyweight Buatsi made a winning yet underwhelming debut to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight ten-round bout at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a match where the techniques failed to mesh, with frequent clinching spoiling the action.
The officials had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but what defines champions? They prevail," the victor stated in the ringside as he was greeted with negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers believed Parker, 31, merited the win but the contender endured the second defeat of his 28-fight professional journey.
The Champion Earns Win in Uneventful Affair
Buatsi's progress has plateaued in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of UK boxing, he now stands trying to achieve expectations - maybe in what feels like a last chance - under recent support.
He began cautiously, with the two boxers prepared to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who operated with a protective stance to narrowly win the cleaner periods.
The audience's tolerance diminished because of regular grabbing, with the challenger going down several times during clinches.
The underdog ended stronger, but several stanzas were hard to judge. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent local rivalry fight to secure the European light heavyweight title.
The venue became quiet for a deeply poignant memorial before the fight as a bell ceremony tolled in honor of local hero Hatton, who died at age 46 in September.
It was the premier televised match card in the location since Hatton's passing. As the concluding sound disappeared, the silence became a roar of "only one Ricky Hatton".
The action inside the ropes reflected the emotion. The eventual winner was floored by a left hook squarely on the jaw in the early round, getting up unsteadily to his position on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged early, significant bruising forming under his face as the duo exchanged hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with powerful upward punches as the action hardly diminished over the full distance.
The victor barely secured the verdict with official scores of 114-114, 115-113 and 115-112, but the two athletes were persistent and courageous, exactly what their inspiration the legend embodied.
