UK light heavyweight Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a competitive ten-round contest at the Manchester venue.
But Parker found success, landing some telling clean blows in a match where the approaches failed to mesh, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my finest showing but how do champions respond? They prevail," the victor stated in the arena as he was met by disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Several boxing experts felt Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter experienced the second loss of his 28-match career.
The Champion Claims Victory in Slow Encounter
His career development has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under recent support.
He started carefully, with both competitors prepared to be aggressive in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who operated with a tight guard to narrowly win the better periods.
The crowd's patience decreased during frequent holding, with Parker falling to the floor several times during clinches.
Parker ended stronger, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Hatton Tribute Inspires All-Manchester Fight
In the co-main event, Arthur beat Rea by close verdict in a excellent all-Manchester fight to secure the European light heavyweight title.
The arena grew hushed for a extremely emotional memorial before the fight as ten bells tolled in honor of Manchester's own Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the final chime faded, the silence transitioned to a roar of "only one Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was floored by a left hook directly to the jaw in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was marked up soon, heavy swelling appearing under his orbital area as the pair traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor narrowly won the verdict with official scores of even score, narrow lead and wider gap, but both fighters were relentless and full of heart, exactly what their idol the legend stood for.
