British light-heavyweight Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a match where the techniques didn't complement each other, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest performance but what do champions do? They prevail," the victor said in the ring as he was met by disapproval from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Some ringside observers thought Parker, 31, should have been awarded the victory but the contender endured the second loss of his 28-fight career.
The Champion Secures Victory in Dull Match
His career development has slowed in recent times through contract problems and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under fresh management.
He began cautiously, with the two boxers prepared to overcommit in a cagey opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who worked behind a solid defense to edge the better stanzas.
The crowd's patience diminished amid constant clinching, with Parker going down several times during clinches.
The underdog closed well, but many rounds were hard to judge. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Motivates All-Manchester Fight
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a excellent all-Manchester contest to win the European light heavyweight belt.
The venue grew hushed for a extremely emotional homage prior to the bout as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound ended, the quiet gave way to a chant of "there's just one Hatton".
The fighting during the bout reflected the emotion. The eventual winner was floored by a left hook squarely on the face in the initial stage, rising shakily to his footing on the count of eight.
But the traditional red attire of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was injured soon, noticeable damage forming under his face as the fighters traded powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur barely secured the result with judges scorecards of even score, 115-113 and clearer margin, but each competitor were determined and courageous, everything their idol the legend stood for.
