UK light-heavyweight Buatsi earned a win yet disappointing debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling individual punches in a fight where the approaches never quite clicked, with excessive holding disrupting the action.
The officials had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my finest performance but what do champions do? They succeed," the victor commented in the arena as he was greeted with negative reactions from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts thought the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter experienced the second setback of his 28-fight professional journey.
The Champion Earns Win in Uneventful Encounter
Buatsi's progress has plateaued in recent years through management issues and long spells of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with both competitors ready to take risks in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who worked behind a tight guard to barely take the more effective periods.
The spectators' acceptance wore thin because of regular grabbing, with the challenger falling to the floor several times during clinches.
Parker closed well, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a excellent local rivalry battle to win the European light-heavyweight belt.
The venue fell silent for a profoundly moving memorial before the fight as ten bells rang in honor of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast fight night in the city since Hatton's death. As the concluding sound faded, the quiet became a chant of "only one Ricky Hatton".
The action inside the ropes matched the emotion. Arthur was knocked down by a left hook squarely on the chin in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was injured early, noticeable damage appearing under his face as the fighters traded punishing blows.
Rea, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur narrowly won the verdict with scoring tallies of even score, 115-113 and 115-112, but the two athletes were relentless and courageous, everything their inspiration the legend embodied.
