British light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round bout at the boxing arena.
But Parker found success, delivering effective single shots in a match where the techniques never quite clicked, with excessive holding disrupting the fight.
Two judges judged it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They win," the victor commented in the arena as he was greeted with disapproval from the crowd.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers believed the challenger, 31, deserved the decision but the British boxer endured the second defeat of his 28-fight professional journey.
The Champion Secures Win in Dull Match
Buatsi's progress has stalled in the past few years through management issues and extended periods of inactivity.
Once tipped as the next star of British boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers willing to take risks in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to barely take the cleaner periods.
The crowd's patience diminished during constant clinching, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
The victor moves on, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires All-Manchester Fight
In the supporting bout, Arthur overcame Rea by divided decision in a excellent all-Manchester fight to capture the continental championship.
The arena grew hushed for a profoundly moving homage prior to the bout as 10 bells tolled in memory of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the urban center since Hatton's passing. As the final chime ended, the silence became a roar of "only one Hatton".
The action within the ring equaled the feeling. The eventual winner was floored by a powerful punch directly to the face in the initial stage, rising shakily to his feet on the referee's count.
But the red-colored attire of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was marked up soon, significant bruising forming under his orbital area as the duo landed punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur edged the verdict with judges scorecards of even score, close margin and clearer margin, but the two athletes were determined and courageous, exactly what their hero the legend represented.
