UK light-heavyweight Buatsi earned a win yet underwhelming start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the styles never quite clicked, with frequent clinching marring the fight.
The officials had it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best showing but what defines champions? They win," Buatsi commented in the ring as he was met by disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to secure a 20th pro success.
Some ringside observers felt the challenger, thirty-one, merited the decision but the Derbyshire fighter experienced the second defeat of his 28-fight career.
The Champion Claims Triumph in Dull Affair
Buatsi's progress has plateaued in recent times through contract problems and long spells of inactivity.
Once tipped as the next star of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under recent support.
He started carefully, with both competitors willing to take risks in a tentative initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who operated with a tight guard to barely take the better stanzas.
The crowd's patience diminished amid frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the co-main event, Arthur beat Rea by divided decision in a excellent Manchester vs Manchester contest to win the continental championship.
The stadium grew hushed for a extremely emotional memorial prior to the bout as 10 bells sounded in honor of local hero Ricky Hatton, who passed away at age 46 in recently.
It was the premier televised match card in the location since Hatton's passing. As the final chime ended, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The competition during the bout equaled the sentiment. The eventual winner was knocked down by a left hook flush on the chin in the early round, rising shakily to his feet on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was damaged quickly, significant bruising developing under his face as the fighters exchanged powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the action rarely dropped over 12 rounds.
Arthur narrowly won the decision with scoring tallies of even score, close margin and wider gap, but each competitor were persistent and courageous, all the qualities their inspiration the legend represented.
