UK light-heavyweight Buatsi made a winning yet unimpressive start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful individual punches in a contest where the approaches failed to mesh, with constant grabbing disrupting the fight.
Two judges scored it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They win," Buatsi said in the ring as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers felt Parker, 31, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-match professional journey.
The Champion Claims Win in Slow Affair
His career development has stalled in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers willing to overcommit in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at light-heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for the favorite, who operated with a solid defense to barely take the cleaner stanzas.
The spectators' acceptance decreased amid regular grabbing, with Parker slipping to the canvas several times during clinches.
Parker finished the stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the caliber of power puncher Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent all-Manchester fight to capture the European light-heavyweight title.
The stadium grew hushed for a deeply poignant memorial prior to the bout as 10 bells rang in honor of local hero Ricky Hatton, who died 46 years old in recently.
It was the premier televised match card in the location since Hatton's death. As the concluding sound faded, the stillness transitioned to a roar of "only one Hatton".
The fighting inside the ropes matched the feeling. Arthur was floored by a left hook squarely on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the red-colored equipment of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was marked up soon, heavy swelling developing under his eye as the pair traded powerful shots.
The challenger, 27, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the intensity seldom decreased over 12 rounds.
The victor narrowly won the decision with scoring tallies of draw, close margin and wider gap, but each competitor were persistent and brave, exactly what their hero Hatton represented.
