UK light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive ten-round fight at the boxing arena.
But the challenger found success, landing some telling single shots in a match where the techniques never quite clicked, with constant grabbing marring the action.
The scoring judges had it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best showing but what do champions do? They succeed," Buatsi stated in the ringside as he was met by disapproval from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts felt the challenger, 31, merited the win but the contender experienced the second loss of his 28-match professional journey.
The Champion Earns Victory in Slow Encounter
Buatsi's progress has slowed in recent times through contract problems and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now stands trying to realize his potential - perhaps in what appears to be a last chance - under fresh management.
He opened tentatively, with both competitors ready to overcommit in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second bout at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a tight guard to edge the better stanzas.
The spectators' acceptance decreased because of constant clinching, with Parker going down multiple occasions during close exchanges.
The underdog finished the stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Hatton Memorial Fuels Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a terrific all-Manchester battle to secure the European light-heavyweight championship.
The venue grew hushed for a deeply poignant homage before the fight as ten bells sounded in memory of local hero Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the last bell disappeared, the stillness gave way to a chant of "there's only one Hatton".
The action within the ring reflected the feeling. Arthur was sent to the canvas by a significant blow flush on the jaw in the initial stage, rising shakily to his feet on the count of eight.
But the red-colored equipment of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was damaged soon, significant bruising appearing under his face as the fighters exchanged powerful shots.
The challenger, 27, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur edged the result with scoring tallies of even score, close margin and wider gap, but both fighters were determined and brave, everything their idol the legend represented.
