British light-heavyweight Buatsi secured a victory yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the techniques failed to mesh, with constant grabbing spoiling the contest.
Two judges scored it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest performance but what do champions do? They succeed," Buatsi commented in the ring as he was met by disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts believed Parker, 31, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-fight professional journey.
The Champion Secures Win in Dull Encounter
Buatsi's progress has plateaued in recent times through contract problems and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter willing to overcommit in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to edge the cleaner rounds.
The audience's tolerance decreased because of constant clinching, with the challenger falling to the floor several times during grappling.
Parker closed well, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the presence of power puncher Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Tribute Inspires All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a terrific Manchester vs Manchester contest to win the European light heavyweight belt.
The venue grew hushed for a extremely emotional homage ahead of the contest as ten bells sounded in remembrance of local hero Ricky Hatton, who succumbed 46 years old in September.
It was the premier televised boxing event in the location since Hatton's death. As the last bell ended, the quiet became a roar of "there's only one Ricky Hatton".
The competition inside the ropes reflected the sentiment. Arthur was sent to the canvas by a left hook flush on the chin in the early round, getting up unsteadily to his footing on the count of eight.
But the traditional red attire of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged early, significant bruising forming under his orbital area as the duo landed punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace rarely dropped over the full distance.
Arthur barely secured the verdict with official scores of draw, narrow lead and 115-112, but each competitor were relentless and courageous, everything their hero Hatton embodied.
