UK light heavyweight Buatsi made a winning yet underwhelming debut to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight ten-round contest at the boxing arena.
But Parker landed significant shots, delivering effective single shots in a match where the techniques never quite clicked, with excessive holding spoiling the contest.
The officials judged it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but what do champions do? They win," Buatsi said in the arena as he was faced disapproval from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a 20th pro success.
Various analysts believed the challenger, 31, deserved the win but the contender endured the second setback of his 28-match career.
The Champion Secures Victory in Dull Encounter
The fighter's advancement has plateaued in the past few years through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what feels like a last chance - under new promotional backing.
He started carefully, with neither fighter willing to take risks in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a tight guard to narrowly win the cleaner stanzas.
The audience's tolerance decreased because of regular grabbing, with the challenger slipping to the canvas multiple occasions during grappling.
The underdog closed well, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Hatton Tribute Fuels Local Rivalry Battle
In the supporting bout, Arthur defeated Rea by close verdict in a excellent all-Manchester fight to win the European light-heavyweight belt.
The arena grew hushed for a extremely emotional tribute ahead of the contest as ten bells sounded in remembrance of the city's native Ricky Hatton, who died at age 46 in recently.
It was the first major televised boxing event in the location since Hatton's passing. As the concluding sound faded, the stillness gave way to a chant of "there's only one Ricky Hatton".
The action inside the ropes matched the sentiment. The eventual winner was floored by a powerful punch flush on the face in the early round, rising shakily to his feet on the referee's count.
But the traditional red attire of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was marked up early, heavy swelling developing under his orbital area as the pair traded hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over the full distance.
Arthur barely secured the result with judges scorecards of draw, narrow lead and 115-112, but both fighters were persistent and brave, all the qualities their hero the legend represented.
