UK light-heavyweight Buatsi secured a victory yet unimpressive start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a fight where the styles didn't complement each other, with excessive holding marring the action.
The officials had it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," Buatsi commented in the ringside as he was met by disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Various analysts believed the challenger, thirty-one, should have been awarded the win but the contender endured the second defeat of his 28-match professional journey.
Buatsi Earns Triumph in Slow Match
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of lack of fights.
Once tipped as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what feels like a last chance - under new promotional backing.
He opened tentatively, with the two boxers willing to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who worked behind a solid defense to edge the better periods.
The crowd's patience wore thin because of regular grabbing, with Parker falling to the floor several times during grappling.
Parker ended stronger, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor moves on, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the supporting bout, Arthur beat Rea by divided decision in a outstanding all-Manchester fight to secure the continental title.
The venue fell silent for a deeply poignant tribute before the fight as 10 bells sounded in remembrance of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the premier televised boxing event in the city since the passing of Hatton. As the last bell disappeared, the stillness gave way to a shout of "only one Ricky Hatton".
The action inside the ropes reflected the emotion. Arthur was sent to the canvas by a significant blow squarely on the jaw in the second round, rising shakily to his position on the eight count.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was injured early, significant bruising appearing under his face as the duo landed punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action seldom decreased over the full distance.
Arthur barely secured the result with scoring tallies of draw, narrow lead and 115-112, but both fighters were persistent and full of heart, exactly what their hero Hatton stood for.
