UK light heavyweight Buatsi secured a victory yet unimpressive start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight ten-round contest at the boxing arena.
But Parker found success, connecting with powerful clean blows in a fight where the styles never quite clicked, with constant grabbing disrupting the contest.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my best performance but how do champions respond? They win," the victor said in the ringside as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to claim a 20th career victory.
Various analysts believed the challenger, 31, deserved the win but the contender suffered the second loss of his 28-bout career.
Buatsi Earns Win in Uneventful Match
His career development has slowed in recent times through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers ready to overcommit in a cagey opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after damaging his hand in the contest, was making just his second bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who worked behind a solid defense to edge the cleaner rounds.
The crowd's patience decreased amid frequent holding, with the challenger going down several times during grappling.
Parker ended stronger, but several stanzas were hard to judge. However, former world champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Homage Motivates Manchester Derby Contest
In the co-main event, Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to secure the continental championship.
The venue grew hushed for a profoundly moving homage before the fight as a bell ceremony tolled in honor of Manchester's own Hatton, who passed away at age 46 in last month.
It was the premier televised boxing event in the urban center since Hatton's death. As the concluding sound faded, the stillness gave way to a shout of "there's only one Ricky Hatton".
The action inside the ropes matched the feeling. Arthur was sent to the canvas by a significant blow directly to the face in the early round, rising shakily to his feet on the count of eight.
But the traditional red shorts of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was marked up soon, significant bruising forming under his orbital area as the pair exchanged punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over the full distance.
The victor barely secured the decision with official scores of even score, close margin and 115-112, but both fighters were relentless and courageous, all the qualities their hero the legend represented.
