UK light heavyweight Buatsi secured a victory yet underwhelming debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive 10-round bout at the boxing arena.
But Parker had his moments, connecting with powerful single shots in a match where the styles failed to mesh, with excessive holding disrupting the contest.
Two judges had it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest performance but what do champions do? They win," Buatsi said in the ring as he was met by disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts felt Parker, thirty-one, deserved the decision but the Derbyshire fighter endured the second defeat of his 28-fight professional journey.
The Champion Secures Triumph in Dull Affair
The fighter's advancement has plateaued in recent times through management issues and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now stands trying to achieve expectations - possibly in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter ready to take risks in a calculated opening few rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to narrowly win the cleaner stanzas.
The spectators' acceptance diminished amid regular grabbing, with the challenger going down various instances during clinches.
Parker finished the stronger, but many rounds were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a excellent Manchester vs Manchester battle to win the continental title.
The arena became quiet for a extremely emotional memorial ahead of the contest as 10 bells sounded in remembrance of local hero Ricky Hatton, who passed away at age 46 in September.
It was the initial significant broadcast match card in the city since Hatton's death. As the concluding sound ended, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The action within the ring equaled the feeling. Arthur was floored by a powerful punch flush on the chin in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up soon, heavy swelling developing under his eye as the fighters traded powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur narrowly won the decision with scoring tallies of even score, 115-113 and 115-112, but each competitor were persistent and full of heart, exactly what their idol the legend represented.
