British light-heavyweight Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a close 10-round contest at the Manchester venue.
But Parker had his moments, delivering effective individual punches in a match where the approaches failed to mesh, with constant grabbing spoiling the contest.
The officials had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my best performance but what do champions do? They succeed," the victor stated in the ringside as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, merited the decision but the British boxer suffered the second setback of his 28-bout career.
Buatsi Secures Triumph in Slow Match
The fighter's advancement has plateaued in the past few years through management issues and considerable time of inactivity.
Once tipped as the future of British boxing, he now stands trying to realize his potential - maybe in what feels like a last chance - under recent support.
He started carefully, with neither fighter willing to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who utilized a tight guard to barely take the more effective periods.
The audience's tolerance decreased during constant clinching, with the challenger falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Hatton Memorial Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding Manchester vs Manchester battle to secure the European light-heavyweight belt.
The arena grew hushed for a profoundly moving tribute before the fight as ten bells rang in remembrance of local hero Hatton, who succumbed at age 46 in September.
It was the first major televised boxing event in the urban center since Hatton's death. As the concluding sound disappeared, the silence became a shout of "there's only one Ricky Hatton".
The fighting within the ring equaled the feeling. The eventual winner was knocked down by a left hook directly to the face in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was marked up soon, heavy swelling appearing under his eye as the fighters exchanged hard punches.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the action rarely dropped over 12 rounds.
The victor barely secured the decision with official scores of draw, narrow lead and wider gap, but each competitor were determined and full of heart, exactly what their hero Hatton embodied.
