UK light-heavyweight Buatsi made a winning yet underwhelming start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a competitive 10-round contest at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a match where the techniques failed to mesh, with excessive holding disrupting the fight.
Two judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi commented in the ring as he was met by disapproval from the crowd.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts felt the challenger, 31, merited the victory but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Secures Victory in Slow Affair
His career development has stalled in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to achieve expectations - maybe in what feels like a last chance - under fresh management.
He began cautiously, with neither fighter ready to overcommit in a tentative initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second fight at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who operated with a solid defense to barely take the better periods.
The spectators' acceptance decreased because of frequent holding, with the challenger falling to the floor various instances during clinches.
Parker closed well, but multiple periods were difficult to score. Although, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester contest to capture the European light heavyweight title.
The stadium became quiet for a extremely emotional tribute prior to the bout as ten bells sounded in memory of Manchester's own Ricky Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime disappeared, the silence transitioned to a chant of "there's just one Ricky Hatton".
The competition within the ring equaled the feeling. The eventual winner was floored by a left hook directly to the jaw in the second round, rising shakily to his feet on the referee's count.
But the traditional red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up early, heavy swelling developing under his orbital area as the pair landed hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor edged the verdict with scoring tallies of even score, close margin and 115-112, but both fighters were persistent and full of heart, everything their hero Hatton stood for.
