UK light-heavyweight Buatsi earned a win yet underwhelming beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a contest where the techniques never quite clicked, with constant grabbing disrupting the action.
The scoring judges had it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my finest showing but what defines champions? They prevail," the victor stated in the arena as he was faced negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Various analysts believed Parker, thirty-one, should have been awarded the decision but the contender experienced the second defeat of his 28-fight career.
The Champion Secures Win in Dull Affair
Buatsi's progress has slowed in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to realize his potential - maybe in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter ready to overcommit in a cagey opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the cleaner rounds.
The audience's tolerance diminished during frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog closed well, but multiple periods were hard to judge. Although, former world champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester contest to secure the European light-heavyweight championship.
The arena fell silent for a deeply poignant homage ahead of the contest as a bell ceremony rang in remembrance of local hero Ricky Hatton, who died at age 46 in recently.
It was the first major televised boxing event in the location since Hatton's death. As the concluding sound disappeared, the silence became a chant of "there's just one Ricky Hatton".
The fighting within the ring matched the sentiment. The eventual winner was floored by a significant blow directly to the chin in the early round, getting up unsteadily to his footing on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was injured early, noticeable damage forming under his face as the duo traded powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over the full distance.
Arthur narrowly won the result with official scores of draw, close margin and 115-112, but both fighters were determined and full of heart, everything their idol the legend stood for.
