British light-heavyweight Buatsi made a winning yet unimpressive start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a tight ten-round fight at the boxing arena.
But Parker found success, connecting with powerful clean blows in a fight where the approaches didn't complement each other, with constant grabbing spoiling the action.
Two judges scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my finest showing but how do champions respond? They succeed," the victor said in the ring as he was met by negative reactions from the audience.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts felt Parker, thirty-one, should have been awarded the win but the contender endured the second setback of his 28-bout career.
The Champion Earns Triumph in Slow Match
Buatsi's progress has stalled in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors willing to overcommit in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who operated with a solid defense to narrowly win the cleaner periods.
The crowd's patience diminished during constant clinching, with Parker slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Hatton Tribute Motivates Local Rivalry Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the European light-heavyweight championship.
The arena fell silent for a profoundly moving tribute ahead of the contest as 10 bells tolled in remembrance of local hero Ricky Hatton, who died at age 46 in recently.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the last bell faded, the stillness transitioned to a roar of "there's just one Hatton".
The competition within the ring equaled the emotion. The eventual winner was knocked down by a left hook squarely on the jaw in the early round, getting up unsteadily to his feet on the referee's count.
But the red-colored shorts of the fighter soon came forward again. Across the ring, the opponent - in competitive blue - was injured early, noticeable damage developing under his orbital area as the fighters landed punishing blows.
The challenger, 27, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over 12 rounds.
Arthur edged the verdict with judges scorecards of even score, 115-113 and 115-112, but the two athletes were persistent and courageous, exactly what their hero the legend embodied.
