British light heavyweight Buatsi made a winning yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive 10-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling clean blows in a fight where the styles didn't complement each other, with excessive holding disrupting the contest.
The officials scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my best performance but how do champions respond? They prevail," the victor said in the arena as he was faced boos from the spectators.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Various analysts felt the challenger, thirty-one, merited the victory but the British boxer endured the second defeat of his 28-fight career.
The Champion Claims Triumph in Uneventful Affair
Buatsi's progress has stalled in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors ready to overcommit in a calculated initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to barely take the cleaner rounds.
The crowd's patience wore thin during regular grabbing, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but multiple periods were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Inspires All-Manchester Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester battle to capture the European light heavyweight belt.
The stadium fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the first major televised fight night in the urban center since Hatton's passing. As the concluding sound ended, the stillness transitioned to a roar of "there's just one Ricky Hatton".
The action during the bout matched the emotion. The eventual winner was floored by a powerful punch flush on the chin in the initial stage, rising shakily to his feet on the eight count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was marked up early, heavy swelling appearing under his eye as the fighters landed powerful shots.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with powerful upward punches as the pace seldom decreased over the full distance.
Arthur narrowly won the verdict with official scores of draw, narrow lead and wider gap, but each competitor were persistent and brave, exactly what their inspiration Hatton represented.
