UK light heavyweight Buatsi earned a win yet underwhelming debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a fight where the styles didn't complement each other, with excessive holding disrupting the fight.
Two judges had it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my best display but how do champions respond? They succeed," the victor said in the arena as he was met by negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th pro success.
Some ringside observers felt the challenger, 31, deserved the win but the contender endured the second defeat of his 28-bout professional journey.
The Champion Earns Victory in Dull Affair
Buatsi's progress has stalled in recent times through contract problems and long spells of inactivity.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under fresh management.
He began cautiously, with the two boxers prepared to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who operated with a tight guard to narrowly win the more effective stanzas.
The spectators' acceptance diminished because of regular grabbing, with Parker going down multiple occasions during grappling.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester fight to win the European light heavyweight title.
The arena fell silent for a extremely emotional tribute ahead of the contest as ten bells rang in memory of the city's native Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the final chime disappeared, the quiet gave way to a roar of "there's only one Ricky Hatton".
The fighting inside the ropes matched the feeling. Arthur was sent to the canvas by a powerful punch directly to the chin in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his orbital area as the pair traded powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur narrowly won the decision with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were determined and full of heart, all the qualities their idol Hatton embodied.
