UK light-heavyweight Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a tight 10-round contest at the Manchester venue.
But the challenger found success, delivering effective single shots in a contest where the styles failed to mesh, with constant grabbing spoiling the contest.
Two judges scored it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They prevail," Buatsi said in the ring as he was met by negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to secure a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the victory but the contender experienced the second defeat of his 28-bout career.
Buatsi Earns Triumph in Uneventful Affair
Buatsi's progress has slowed in the past few years through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to take risks in a calculated opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who worked behind a protective stance to narrowly win the cleaner rounds.
The crowd's patience diminished because of constant clinching, with Parker going down various instances during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Tribute Motivates Manchester Derby Battle
In the secondary fight, Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester fight to secure the European light heavyweight title.
The stadium fell silent for a deeply poignant tribute prior to the bout as 10 bells sounded in memory of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound faded, the silence transitioned to a roar of "only one Ricky Hatton".
The competition inside the ropes matched the emotion. Arthur was floored by a significant blow flush on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was injured quickly, noticeable damage forming under his orbital area as the pair traded hard punches.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the action seldom decreased over the full distance.
Arthur barely secured the result with judges scorecards of 114-114, 115-113 and 115-112, but the two athletes were persistent and courageous, all the qualities their inspiration the legend represented.
