UK light-heavyweight Buatsi earned a win yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight 10-round bout at the boxing arena.
But the challenger found success, connecting with powerful clean blows in a match where the styles never quite clicked, with frequent clinching marring the action.
The officials had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best performance but what do champions do? They prevail," the victor commented in the ringside as he was faced disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to earn a 20th career victory.
Various analysts believed the challenger, thirty-one, merited the win but the Derbyshire fighter experienced the second defeat of his 28-fight career.
Buatsi Earns Victory in Dull Match
His career development has slowed in recent times through contract problems and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who utilized a protective stance to edge the better rounds.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific Manchester vs Manchester contest to capture the European light heavyweight belt.
The venue became quiet for a deeply poignant homage prior to the bout as a bell ceremony rang in memory of Manchester's own Hatton, who passed away aged 46 in recently.
It was the first major televised fight night in the location since Hatton's passing. As the final chime faded, the silence became a chant of "there's just one Ricky Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was knocked down by a powerful punch squarely on the jaw in the second round, getting up unsteadily to his feet on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged quickly, significant bruising forming under his eye as the pair exchanged hard punches.
The challenger, 27, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the action hardly diminished over the full distance.
Arthur edged the verdict with judges scorecards of draw, close margin and clearer margin, but each competitor were persistent and courageous, all the qualities their inspiration the legend stood for.
