UK light heavyweight Buatsi secured a victory yet unimpressive start to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a competitive 10-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective single shots in a match where the techniques didn't complement each other, with frequent clinching marring the action.
The officials scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest performance but how do champions respond? They succeed," the victor said in the ring as he was faced negative reactions from the audience.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts thought Parker, 31, deserved the victory but the contender endured the second defeat of his 28-match professional journey.
The Champion Claims Victory in Dull Encounter
The fighter's advancement has slowed in the past few years through management issues and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers prepared to take risks in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to barely take the better stanzas.
The crowd's patience decreased amid regular grabbing, with the challenger going down several times during close exchanges.
Parker closed well, but multiple periods were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a terrific local rivalry contest to secure the European light heavyweight title.
The arena became quiet for a extremely emotional memorial before the fight as 10 bells rang in honor of the city's native Hatton, who died 46 years old in September.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the concluding sound disappeared, the silence gave way to a chant of "there's only one Hatton".
The fighting during the bout equaled the feeling. Arthur was sent to the canvas by a left hook directly to the jaw in the second round, rising shakily to his feet on the count of eight.
But the traditional red equipment of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was marked up soon, noticeable damage developing under his face as the pair landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the result with scoring tallies of even score, 115-113 and clearer margin, but each competitor were persistent and brave, exactly what their inspiration the legend represented.
