British light-heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close ten-round bout at the boxing arena.
But the challenger landed significant shots, landing some telling individual punches in a match where the approaches never quite clicked, with frequent clinching disrupting the action.
Two judges judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest display but what defines champions? They succeed," the victor commented in the ringside as he was met by negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Some ringside observers felt the challenger, 31, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-fight career.
The Champion Secures Triumph in Slow Encounter
Buatsi's progress has stalled in the past few years through management issues and long spells of lack of fights.
Once tipped as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He began cautiously, with both competitors willing to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who operated with a tight guard to narrowly win the cleaner rounds.
The audience's tolerance decreased during regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
The underdog ended stronger, but multiple periods were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a terrific Manchester vs Manchester fight to capture the European light heavyweight championship.
The venue grew hushed for a profoundly moving tribute before the fight as a bell ceremony sounded in remembrance of local hero Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell disappeared, the quiet gave way to a shout of "there's only one Ricky Hatton".
The competition during the bout equaled the feeling. Arthur was sent to the canvas by a left hook flush on the face in the early round, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was marked up early, significant bruising forming under his orbital area as the fighters landed punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor edged the result with judges scorecards of draw, narrow lead and clearer margin, but both fighters were determined and brave, exactly what their hero the legend represented.
