UK light-heavyweight Buatsi secured a victory yet disappointing debut to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a competitive 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a fight where the techniques failed to mesh, with excessive holding marring the contest.
Two judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest showing but how do champions respond? They prevail," Buatsi commented in the ring as he was faced boos from the crowd.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the decision but the contender experienced the second setback of his 28-bout professional journey.
Buatsi Earns Win in Uneventful Match
His career development has plateaued in recent times through contract problems and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who worked behind a tight guard to barely take the cleaner periods.
The crowd's patience diminished amid constant clinching, with Parker slipping to the canvas various instances during clinches.
Parker ended stronger, but several stanzas were difficult to score. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Homage Inspires All-Manchester Fight
In the supporting bout, Arthur overcame Bradley Rea by split-decision in a terrific local rivalry fight to capture the European light-heavyweight title.
The arena became quiet for a extremely emotional memorial before the fight as ten bells sounded in remembrance of Manchester's own Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the final chime ended, the quiet became a chant of "there's only one Ricky Hatton".
The action inside the ropes reflected the feeling. The eventual winner was knocked down by a significant blow directly to the jaw in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was injured early, heavy swelling forming under his eye as the pair traded powerful shots.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor narrowly won the decision with official scores of draw, narrow lead and 115-112, but both fighters were relentless and courageous, everything their idol the legend stood for.
