British light heavyweight Buatsi made a winning yet disappointing debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a close 10-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful single shots in a fight where the techniques didn't complement each other, with frequent clinching spoiling the action.
The officials had it 96-94 for to the winner, with the final judge a even score.
"It wasn't my finest showing but how do champions respond? They succeed," the victor commented in the arena as he was met by boos from the crowd.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers thought the challenger, 31, deserved the win but the contender endured the second defeat of his 28-fight career.
Buatsi Secures Victory in Slow Affair
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers ready to be aggressive in a tentative opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to barely take the cleaner periods.
The spectators' acceptance diminished because of constant clinching, with the challenger slipping to the canvas multiple occasions during grappling.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Motivates Manchester Derby Battle
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a outstanding local rivalry contest to secure the continental belt.
The stadium fell silent for a extremely emotional tribute prior to the bout as 10 bells rang in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the premier televised match card in the urban center since Hatton's death. As the concluding sound disappeared, the stillness transitioned to a roar of "there's only one Hatton".
The action within the ring matched the emotion. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the second round, getting up unsteadily to his position on the referee's count.
But the red-colored attire of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his orbital area as the duo exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the result with scoring tallies of 114-114, close margin and wider gap, but each competitor were determined and courageous, all the qualities their hero Hatton stood for.
