UK light heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a match where the techniques failed to mesh, with excessive holding spoiling the action.
Two judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what do champions do? They succeed," Buatsi stated in the arena as he was greeted with boos from the crowd.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Various analysts thought Parker, thirty-one, deserved the win but the Derbyshire fighter endured the second loss of his 28-fight career.
The Champion Earns Triumph in Slow Match
His career development has slowed in recent times through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to realize his potential - maybe in what appears to be a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who operated with a protective stance to barely take the better rounds.
The crowd's patience diminished during regular grabbing, with the challenger going down various instances during clinches.
Parker closed well, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Inspires All-Manchester Fight
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester battle to capture the European light heavyweight championship.
The stadium grew hushed for a extremely emotional tribute ahead of the contest as 10 bells sounded in memory of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound ended, the stillness became a chant of "there's just one Ricky Hatton".
The action within the ring reflected the emotion. The eventual winner was knocked down by a powerful punch directly to the face in the second round, getting up unsteadily to his footing on the eight count.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was injured early, heavy swelling appearing under his face as the pair exchanged punishing blows.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the pace hardly diminished over the full distance.
Arthur narrowly won the verdict with judges scorecards of 114-114, narrow lead and wider gap, but both fighters were determined and full of heart, exactly what their hero the legend embodied.
