British light-heavyweight Buatsi earned a win yet disappointing beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a split decision in a competitive ten-round bout at the boxing arena.
But the challenger found success, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with constant grabbing disrupting the contest.
Two judges scored it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best performance but how do champions respond? They win," Buatsi stated in the ringside as he was met by disapproval from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts thought Parker, 31, merited the win but the British boxer endured the second setback of his 28-bout career.
The Champion Secures Victory in Dull Match
Buatsi's progress has stalled in recent times through contract problems and long spells of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers ready to take risks in a calculated early stages.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second bout at light-heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who utilized a solid defense to edge the cleaner rounds.
The crowd's patience diminished amid constant clinching, with the challenger falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but many rounds were challenging to assess. However, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Hatton Homage Inspires Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding local rivalry contest to win the European light-heavyweight title.
The stadium fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony tolled in honor of Manchester's own Ricky Hatton, who died at age 46 in last month.
It was the premier televised boxing event in the city since Hatton's passing. As the last bell disappeared, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The competition within the ring matched the sentiment. The eventual winner was knocked down by a left hook directly to the jaw in the initial stage, getting up unsteadily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was damaged early, heavy swelling developing under his orbital area as the fighters exchanged powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the pace hardly diminished over the full distance.
Arthur barely secured the result with scoring tallies of 114-114, 115-113 and 115-112, but both fighters were determined and full of heart, exactly what their idol the legend embodied.
