British light heavyweight Buatsi made a winning yet disappointing beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive ten-round contest at the Manchester venue.
But Parker found success, connecting with powerful single shots in a fight where the styles never quite clicked, with frequent clinching spoiling the action.
The officials had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest performance but how do champions respond? They prevail," the victor said in the ring as he was greeted with negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers believed Parker, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
The Champion Claims Victory in Dull Encounter
The fighter's advancement has stalled in recent times through promotional disputes and long spells of inactivity.
Once tipped as the future of UK boxing, he now stands trying to realize his potential - maybe in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors ready to be aggressive in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to barely take the cleaner stanzas.
The crowd's patience decreased during constant clinching, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for the challenger, 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 road to world glory - based on this performance - still appears challenging.
Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry contest to win the European light-heavyweight championship.
The stadium fell silent for a profoundly moving memorial ahead of the contest as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime faded, the quiet became a roar of "only one Ricky Hatton".
The competition during the bout reflected the feeling. The eventual winner was floored by a significant blow squarely on the face in the early round, getting up unsteadily to his position on the referee's count.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged early, heavy swelling forming under his face as the duo traded punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur edged the result with scoring tallies of 114-114, narrow lead and wider gap, but both fighters were persistent and brave, everything their inspiration Hatton embodied.
