British light-heavyweight Buatsi made a winning yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round fight at the Co-op Live Arena.
But the challenger had his moments, landing some telling clean blows in a match where the approaches never quite clicked, with frequent clinching disrupting the fight.
The officials judged it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest showing but what do champions do? They succeed," Buatsi said in the arena as he was met by negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, merited the win but the contender suffered the second setback of his 28-match career.
Buatsi Earns Win in Slow Affair
Buatsi's progress has stalled in recent times through management issues and considerable time of lack of fights.
Once tipped as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter willing to take risks in a tentative opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after breaking his hand mid-fight, was making just his second fight at light-heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for Buatsi, who utilized a tight guard to barely take the more effective stanzas.
The audience's tolerance wore thin amid frequent holding, with the challenger going down various instances during grappling.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Contest
In the supporting bout, Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester fight to win the continental title.
The stadium became quiet for a profoundly moving tribute ahead of the contest as ten bells rang in memory of the city's native Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the final chime ended, the silence gave way to a shout of "only one Hatton".
The competition during the bout reflected the sentiment. The eventual winner was sent to the canvas by a left hook directly to the face in the early round, standing uncertainly to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was injured early, significant bruising appearing under his orbital area as the pair landed punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity rarely dropped over twelve rounds.
Arthur edged the decision with official scores of even score, narrow lead and clearer margin, but the two athletes were persistent and courageous, everything their idol Hatton represented.
