British light-heavyweight Buatsi earned a win yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a close ten-round contest at the boxing arena.
But Parker found success, landing some telling single shots in a contest where the approaches never quite clicked, with frequent clinching spoiling the action.
The scoring judges judged it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest performance but what do champions do? They win," the victor stated in the ring as he was greeted with boos from the audience.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Various analysts believed Parker, 31, merited the victory but the British boxer endured the second defeat of his 28-bout career.
The Champion Secures Win in Uneventful Match
The fighter's advancement has plateaued in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the future of British boxing, he now stands trying to realize his potential - possibly in what appears to be a final roll of the dice - under recent support.
He began cautiously, with both competitors ready to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who utilized a tight guard to narrowly win the better periods.
The crowd's patience diminished amid frequent holding, with the challenger falling to the floor multiple occasions during clinches.
Parker ended stronger, but many rounds were difficult to score. However, former world champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the co-main event, Arthur beat Rea by split-decision in a outstanding all-Manchester battle to win the continental belt.
The venue fell silent for a deeply poignant homage ahead of the contest as ten bells tolled in remembrance of the city's native Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the last bell ended, the quiet became a chant of "only one Hatton".
The competition during the bout equaled the emotion. The eventual winner was floored by a left hook squarely on the chin in the second round, standing uncertainly to his position on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was injured soon, heavy swelling developing under his eye as the pair landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur narrowly won the verdict with official scores of 114-114, 115-113 and clearer margin, but each competitor were persistent and brave, all the qualities their hero the legend embodied.
