British light-heavyweight Buatsi secured a victory yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight 10-round fight at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a match where the techniques never quite clicked, with frequent clinching spoiling the contest.
The officials judged it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but what defines champions? They succeed," Buatsi commented in the arena as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, should have been awarded the decision but the contender endured the second setback of his 28-fight career.
The Champion Claims Win in Uneventful Encounter
Buatsi's progress has stalled in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with both competitors ready to take risks in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, making life awkward for the favorite, who operated with a solid defense to narrowly win the better stanzas.
The spectators' acceptance decreased amid frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Homage Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester contest to secure the European light-heavyweight belt.
The arena grew hushed for a extremely emotional memorial ahead of the contest as 10 bells sounded in memory of the city's native Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the urban center since Hatton's passing. As the final chime ended, the silence became a shout of "there's only one Ricky Hatton".
The competition inside the ropes reflected the sentiment. The eventual winner was knocked down by a significant blow squarely on the jaw in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red shorts of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was injured quickly, noticeable damage appearing under his orbital area as the pair traded powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the action hardly diminished over the full distance.
Arthur edged the result with judges scorecards of 114-114, 115-113 and 115-112, but both fighters were relentless and brave, everything their inspiration Hatton stood for.
