British light heavyweight Buatsi made a winning yet disappointing start to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a close ten-round bout at the boxing arena.
But the challenger found success, delivering effective clean blows in a match where the styles never quite clicked, with excessive holding disrupting the action.
Two judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They prevail," Buatsi commented in the ringside as he was met by boos from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts felt Parker, thirty-one, merited the decision but the contender endured the second setback of his 28-fight career.
Buatsi Claims Triumph in Dull Encounter
Buatsi's progress has plateaued in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the rising talent of UK boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to overcommit in a cagey opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who worked behind a tight guard to narrowly win the cleaner periods.
The crowd's patience wore thin during constant clinching, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker closed well, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the presence of hard hitter Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Contest
In the secondary fight, Arthur beat Rea by divided decision in a terrific local rivalry fight to secure the European light-heavyweight belt.
The stadium became quiet for a deeply poignant memorial prior to the bout as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised match card in the location since Hatton's death. As the last bell faded, the stillness gave way to a chant of "only one Ricky Hatton".
The competition inside the ropes equaled the feeling. Arthur was floored by a significant blow flush on the jaw in the second round, standing uncertainly to his feet on the referee's count.
But the red-colored attire of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was injured soon, noticeable damage developing under his eye as the fighters landed powerful shots.
Rea, 27, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the intensity rarely dropped over 12 rounds.
The victor edged the result with judges scorecards of even score, close margin and 115-112, but the two athletes were persistent and brave, all the qualities their hero Hatton stood for.
