UK light heavyweight Buatsi secured a victory yet disappointing start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a close 10-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective single shots in a match where the styles failed to mesh, with excessive holding spoiling the action.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They succeed," Buatsi said in the ringside as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers believed Parker, 31, deserved the victory but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Encounter
Buatsi's progress has slowed in the past few years through management issues and extended periods of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a tentative early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who operated with a tight guard to edge the more effective periods.
The crowd's patience wore thin during regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
Parker closed well, but several stanzas were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the co-main event, Arthur defeated Bradley Rea by divided decision in a terrific all-Manchester contest to capture the continental championship.
The arena grew hushed for a extremely emotional memorial ahead of the contest as ten bells rang in memory of Manchester's own Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the concluding sound faded, the quiet became a roar of "only one Ricky Hatton".
The competition during the bout matched the emotion. The eventual winner was floored by a left hook flush on the chin in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was marked up soon, heavy swelling developing under his face as the fighters traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity seldom decreased over 12 rounds.
Arthur barely secured the decision with official scores of 114-114, 115-113 and clearer margin, but the two athletes were persistent and full of heart, everything their inspiration Hatton represented.
