British light heavyweight Buatsi earned a win yet underwhelming beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a close ten-round bout at the boxing arena.
But Parker had his moments, delivering effective individual punches in a contest where the styles didn't complement each other, with constant grabbing disrupting the contest.
The officials scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my best showing but what do champions do? They win," the victor stated in the ringside as he was faced negative reactions from the crowd.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers believed Parker, 31, should have been awarded the decision but the Derbyshire fighter experienced the second loss of his 28-fight professional journey.
The Champion Secures Victory in Slow Match
His career development has slowed in recent times through promotional disputes and considerable time of lack of fights.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - maybe in what appears to be a last chance - under fresh management.
He started carefully, with the two boxers willing to be aggressive in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second fight at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who operated with a solid defense to edge the better stanzas.
The spectators' acceptance diminished because of frequent holding, with Parker slipping to the canvas several times during grappling.
The underdog closed well, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a outstanding local rivalry contest to capture the European light heavyweight championship.
The stadium fell silent for a profoundly moving tribute ahead of the contest as ten bells rang in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised boxing event in the city since Hatton's passing. As the final chime faded, the stillness gave way to a roar of "there's only one Ricky Hatton".
The competition inside the ropes reflected the emotion. The eventual winner was floored by a left hook directly to the chin in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red attire of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was damaged early, significant bruising developing under his eye as the fighters traded powerful shots.
The challenger, 27, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur edged the result with scoring tallies of draw, 115-113 and wider gap, but the two athletes were determined and full of heart, exactly what their idol the legend represented.
