British light-heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a contest where the approaches failed to mesh, with excessive holding marring the fight.
The officials judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my best showing but what defines champions? They succeed," Buatsi stated in the arena as he was faced disapproval from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Some ringside observers felt Parker, 31, should have been awarded the decision but the contender suffered the second setback of his 28-match career.
The Champion Earns Win in Slow Affair
His career development has stalled in the past few years through management issues and long spells of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a last chance - under fresh management.
He started carefully, with neither fighter willing to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who operated with a tight guard to narrowly win the better stanzas.
The spectators' acceptance diminished because of regular grabbing, with the challenger falling to the floor various instances during clinches.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor advances, but with the presence of power puncher Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the supporting bout, Arthur overcame Rea by close verdict in a outstanding local rivalry contest to capture the European light-heavyweight belt.
The stadium became quiet for a deeply poignant homage before the fight as ten bells sounded in honor of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the premier televised boxing event in the location since Hatton's death. As the final chime ended, the stillness gave way to a roar of "only one Ricky Hatton".
The fighting within the ring equaled the emotion. Arthur was sent to the canvas by a left hook squarely on the chin in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was damaged early, noticeable damage forming under his eye as the fighters exchanged powerful shots.
Rea, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over the full distance.
The victor barely secured the verdict with judges scorecards of even score, 115-113 and wider gap, but the two athletes were relentless and courageous, everything their inspiration the legend embodied.
