UK light heavyweight Buatsi made a winning yet underwhelming beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a competitive ten-round contest at the boxing arena.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the approaches failed to mesh, with frequent clinching spoiling the fight.
The scoring judges judged it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," Buatsi said in the ring as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts believed the challenger, thirty-one, deserved the win but the British boxer experienced the second loss of his 28-bout career.
The Champion Secures Win in Dull Encounter
Buatsi's progress has stalled in recent times through management issues and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to realize his potential - possibly in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with both competitors willing to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who operated with a tight guard to barely take the cleaner periods.
The audience's tolerance diminished because of constant clinching, with the challenger falling to the floor various instances during close exchanges.
The underdog closed well, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Homage Inspires Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a outstanding all-Manchester fight to secure the European light-heavyweight title.
The stadium fell silent for a deeply poignant tribute ahead of the contest as ten bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away 46 years old in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the concluding sound disappeared, the stillness gave way to a roar of "only one Ricky Hatton".
The competition inside the ropes reflected the feeling. Arthur was knocked down by a left hook directly to the jaw in the second round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was marked up soon, significant bruising forming under his eye as the duo landed punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over twelve rounds.
Arthur narrowly won the verdict with official scores of 114-114, close margin and 115-112, but each competitor were persistent and brave, everything their inspiration the legend stood for.
